From 06e6b883917651c50ea97142ae97dbc1c3061cf5 Mon Sep 17 00:00:00 2001
From: Jonathan Apodaca <jrapodaca@gmail.com>
Date: Wed, 11 Jun 2025 20:04:46 -0600
Subject: [PATCH] range: extmarks/tsquery; renderer: text-change

---
 .github/workflows/ci.yaml |   8 +-
 .woodpecker/ci.yaml       |  10 ++
 Makefile                  |   3 +
 examples/filetree.lua     |  28 +--
 examples/form.lua         | 117 +++++++++++++
 examples/notify.lua       |   8 +-
 examples/picker.lua       |  12 +-
 lua/u/buffer.lua          |   5 +-
 lua/u/codewriter.lua      |   2 +-
 lua/u/logger.lua          |  26 +--
 lua/u/pos.lua             |  11 ++
 lua/u/range.lua           | 180 +++++++++++++++----
 lua/u/renderer.lua        | 356 ++++++++++++++++++++++++++++++--------
 lua/u/utils.lua           | 169 ++++++++++++++++--
 shell.nix                 |   7 +-
 spec/range_spec.lua       | 283 +++++++++++++++++++++++++++++-
 spec/renderer_spec.lua    | 137 ++++++++++++++-
 17 files changed, 1188 insertions(+), 174 deletions(-)
 create mode 100644 .woodpecker/ci.yaml
 create mode 100644 examples/form.lua

diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index c8e6c01..55d9ab4 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -14,16 +14,16 @@ jobs:
 
       - name: Populate Nix store
         run:
-          nix-shell --run 'true'
+          nix-shell --pure --run 'true'
 
       - name: Type-check with lua-language-server
         run:
-          nix-shell --run 'make lint'
+          nix-shell --pure --run 'make lint'
 
       - name: Check formatting with stylua
         run:
-          nix-shell --run 'make fmt-check'
+          nix-shell --pure --run 'make fmt-check'
 
       - name: Run busted tests
         run:
-          nix-shell --run 'make test'
+          nix-shell --pure --run 'make test'
diff --git a/.woodpecker/ci.yaml b/.woodpecker/ci.yaml
new file mode 100644
index 0000000..8374cb6
--- /dev/null
+++ b/.woodpecker/ci.yaml
@@ -0,0 +1,10 @@
+when:
+  - event: push
+
+steps:
+  - name: build
+    image: nixos/nix
+    commands:
+      - nix-shell --pure --run 'make lint'
+      - nix-shell --pure --run 'make fmt-check'
+      - nix-shell --pure --run 'make test'
diff --git a/Makefile b/Makefile
index 0c4a074..4e5b84f 100644
--- a/Makefile
+++ b/Makefile
@@ -19,3 +19,6 @@ test:
 	@echo "## Generating coverage report"
 	@luacov
 	@awk '/^Summary$$/{flag=1;next} flag{print}' luacov.report.out
+
+watch:
+	@watchexec -c -e lua make
diff --git a/examples/filetree.lua b/examples/filetree.lua
index d3b3998..ed175b0 100644
--- a/examples/filetree.lua
+++ b/examples/filetree.lua
@@ -10,10 +10,10 @@
 --   change on the underlying filesystem.
 --------------------------------------------------------------------------------
 
---- @alias FsDir { kind: 'dir'; path: string; expanded: boolean; children: FsNode[] }
---- @alias FsFile { kind: 'file'; path: string }
---- @alias FsNode FsDir | FsFile
---- @alias ShowOpts { root_path?: string, width?: number, focus_path?: string }
+--- @alias u.examples.FsDir { kind: 'dir'; path: string; expanded: boolean; children: u.examples.FsNode[] }
+--- @alias u.examples.FsFile { kind: 'file'; path: string }
+--- @alias u.examples.FsNode u.examples.FsDir | u.examples.FsFile
+--- @alias u.examples.ShowOpts { root_path?: string, width?: number, focus_path?: string }
 
 local Buffer = require 'u.buffer'
 local Renderer = require('u.renderer').Renderer
@@ -58,13 +58,13 @@ function H.relative(path, base)
 end
 
 --- @param root_path string
---- @return { tree: FsDir; path_to_node: table<string, FsNode> }
+--- @return { tree: u.examples.FsDir; path_to_node: table<string, u.examples.FsNode> }
 function H.get_tree_inf(root_path)
   logger:info { 'get_tree_inf', root_path }
-  --- @type table<string, FsNode>
+  --- @type table<string, u.examples.FsNode>
   local path_to_node = {}
 
-  --- @type FsDir
+  --- @type u.examples.FsDir
   local tree = {
     kind = 'dir',
     path = H.normalize(root_path or '.'),
@@ -77,8 +77,8 @@ function H.get_tree_inf(root_path)
   return { tree = tree, path_to_node = path_to_node }
 end
 
---- @param tree FsDir
---- @param path_to_node table<string, FsNode>
+--- @param tree u.examples.FsDir
+--- @param path_to_node table<string, u.examples.FsNode>
 function H.populate_dir_children(tree, path_to_node)
   tree.children = {}
 
@@ -135,7 +135,7 @@ local function _render_in_buffer(opts)
       local parts = H.split_path(H.relative(focused_path, tree_inf.tree.path))
       local path_to_node = tree_inf.path_to_node
 
-      --- @param node FsDir
+      --- @param node u.examples.FsDir
       --- @param child_names string[]
       local function expand_to(node, child_names)
         if #child_names == 0 then return end
@@ -310,7 +310,7 @@ local function _render_in_buffer(opts)
   --
   local renderer = Renderer.new(opts.bufnr)
   tracker.create_effect(function()
-    --- @type { tree: FsDir; path_to_node: table<string, FsNode> }
+    --- @type { tree: u.examples.FsDir; path_to_node: table<string, u.examples.FsNode> }
     local tree_inf = s_tree_inf:get()
     local tree = tree_inf.tree
 
@@ -329,7 +329,7 @@ local function _render_in_buffer(opts)
 
     --- Since the filesystem is a recursive tree of nodes, we need to
     --- recursively render each node. This function does just that:
-    --- @param node FsNode
+    --- @param node u.examples.FsNode
     --- @param level number
     local function render_node(node, level)
       local name = vim.fs.basename(node.path)
@@ -414,7 +414,7 @@ end
 local current_inf = nil
 
 --- Show the filetree:
---- @param opts? ShowOpts
+--- @param opts? u.examples.ShowOpts
 function M.show(opts)
   if current_inf ~= nil then return current_inf.controller end
   opts = opts or {}
@@ -456,7 +456,7 @@ function M.hide()
 end
 
 --- Toggle the filetree:
---- @param opts? ShowOpts
+--- @param opts? u.examples.ShowOpts
 function M.toggle(opts)
   if current_inf == nil then
     M.show(opts)
diff --git a/examples/form.lua b/examples/form.lua
new file mode 100644
index 0000000..875e128
--- /dev/null
+++ b/examples/form.lua
@@ -0,0 +1,117 @@
+-- form.lua:
+--
+-- This is a runnable example of a form. Open this file in Neovim, and execute
+-- `:luafile %` to run it. It will create a new buffer to the side, and render
+-- an interactive form. Edit the "inputs" between the `[...]` brackets, and
+-- watch the buffer react immediately to your changes.
+--
+
+local Renderer = require('u.renderer').Renderer
+local h = require('u.renderer').h
+local tracker = require 'u.tracker'
+
+-- Create a new, temporary, buffer to the side:
+vim.cmd.vnew()
+vim.bo.buftype = 'nofile'
+vim.bo.bufhidden = 'wipe'
+vim.bo.buflisted = false
+local renderer = Renderer.new()
+
+-- Create two signals:
+local s_name = tracker.create_signal 'whoever-you-are'
+local s_age = tracker.create_signal 'ideally-a-number'
+
+-- We can create derived information from the signals above. Say we want to do
+-- some validation on the input for `age`: we can do that with a memo:
+local s_age_info = tracker.create_memo(function()
+  local age_raw = s_age:get()
+  local age_digits = age_raw:match '^%s*(%d+)%s*$'
+  local age_n = age_digits and tonumber(age_digits) or nil
+  return {
+    type = age_n and 'number' or 'string',
+    raw = age_raw,
+    n = age_n,
+    n1 = age_n and age_n + 1 or nil,
+  }
+end)
+
+-- This is the render effect that depends on the signals created above. This
+-- will re-run every time one of the signals changes.
+tracker.create_effect(function()
+  local name = s_name:get()
+  local age = s_age:get()
+  local age_info = s_age_info:get()
+
+  -- Each time the signals change, we re-render the buffer:
+  renderer:render {
+    h.Type({}, '# Form Example'),
+    '\n\n',
+
+    -- We can also listen for when specific locations in the buffer change, on
+    -- a tag-by-tag basis. This gives us two-way data-binding between the
+    -- buffer and the signals.
+    {
+      'Name: ',
+      h.Structure({
+        on_change = function(text) s_name:set(text) end,
+      }, name),
+    },
+    {
+      '\nAge: ',
+      h.Structure({
+        on_change = function(text) s_age:set(text) end,
+      }, age),
+    },
+
+    '\n\n',
+
+    -- Show the values of the signals here, too, so that we can see the
+    -- reactivity in action. If you change the values in the tags above, you
+    -- can see the changes reflected here immediately.
+    { 'Hello, "', name, '"!' },
+
+    --
+    -- A more complex example: we can do much more complex rendering, based on
+    -- the state. For example, if you type different values into the `age`
+    -- field, you can see not only the displayed information change, but also
+    -- the color of the highlights in this section will adapt to the type of
+    -- information that has been detected.
+    --
+    -- If string input is detected, values below are shown in the
+    -- `String`/`ErrorMsg` highlight groups.
+    --
+    -- If number input is detected, values below are shown in the `Number`
+    -- highlight group.
+    --
+    -- If a valid number is entered, then this section also displays how old
+    -- you willl be next year (`n + 1`).
+    --
+
+    '\n\n',
+    h.Type({}, '## Computed Information (derived from `age`)'),
+    '\n\n',
+    {
+      'Type: ',
+      h('text', {
+        hl = age_info.type == 'number' and 'Number' or 'String',
+      }, age_info.type),
+    },
+    { '\nRaw input: ', h.String({}, '"' .. age_info.raw .. '"') },
+    {
+      '\nCurrent age: ',
+      age_info.n
+          -- Show the age:
+          and h.Number({}, tostring(age_info.n))
+        -- Show an error-placeholder if the age is invalid:
+        or h.ErrorMsg({}, '(?)'),
+    },
+
+    -- This part is shown conditionally, i.e., only if the age next year can be
+    -- computed:
+    age_info.n1
+      and {
+        '\nAge next year: ',
+        h.Number({}, tostring(age_info.n1)),
+      },
+  }
+end)
diff --git a/examples/notify.lua b/examples/notify.lua
index 869404f..7d289d5 100644
--- a/examples/notify.lua
+++ b/examples/notify.lua
@@ -14,7 +14,7 @@ local ICONS = {
 }
 local DEFAULT_ICON = { text = '', group = 'DiagnosticSignOk' }
 
---- @alias Notification {
+--- @alias u.examples.Notification {
 ---   kind: number;
 ---   id: number;
 ---   text: string;
@@ -30,7 +30,7 @@ local s_notifications = s_notifications_raw:debounce(50)
 
 -- Render effect:
 tracker.create_effect(function()
-  --- @type Notification[]
+  --- @type u.examples.Notification[]
   local notifs = s_notifications:get()
 
   if #notifs == 0 then
@@ -99,14 +99,14 @@ local function my_notify(msg, level, opts)
 
   local id = math.random(math.huge)
 
-  --- @param notifs Notification[]
+  --- @param notifs u.examples.Notification[]
   s_notifications_raw:schedule_update(function(notifs)
     table.insert(notifs, { kind = level, id = id, text = msg })
     return notifs
   end)
 
   vim.defer_fn(function()
-    --- @param notifs Notification[]
+    --- @param notifs u.examples.Notification[]
     s_notifications_raw:schedule_update(function(notifs)
       for i, notif in ipairs(notifs) do
         if notif.id == id then
diff --git a/examples/picker.lua b/examples/picker.lua
index a388733..866d8fc 100644
--- a/examples/picker.lua
+++ b/examples/picker.lua
@@ -44,7 +44,7 @@ local function shallow_copy_arr(arr) return vim.iter(arr):totable() end
 --    shortest portion of this function.
 --------------------------------------------------------------------------------
 
---- @alias SelectController {
+--- @alias u.examples.SelectController {
 ---   get_items: fun(): T[];
 ---   set_items: fun(items: T[]);
 ---   set_filter_text: fun(filter_text: string);
@@ -53,17 +53,17 @@ local function shallow_copy_arr(arr) return vim.iter(arr):totable() end
 ---   set_selected_indices: fun(indicies: number[], ephemeral?: boolean);
 ---   close: fun();
 --- }
---- @alias SelectOpts<T> {
+--- @alias u.examples.SelectOpts<T> {
 ---   items: `T`[];
 ---   multi?: boolean;
----   format_item?: fun(item: T): Tree;
+---   format_item?: fun(item: T): u.renderer.Tree;
 ---   on_finish?: fun(items: T[], indicies: number[]);
 ---   on_selection_changed?: fun(items: T[], indicies: number[]);
----   mappings?: table<string, fun(select: SelectController)>;
+---   mappings?: table<string, fun(select: u.examples.SelectController)>;
 --- }
 
 --- @generic T
---- @param opts SelectOpts<T>
+--- @param opts u.examples.SelectOpts<T>
 function M.create_picker(opts) -- {{{
   local is_in_insert_mode = vim.api.nvim_get_mode().mode:sub(1, 1) == 'i'
   local stopinsert = not is_in_insert_mode
@@ -557,7 +557,7 @@ function M.create_picker(opts) -- {{{
     return safe_run(function() H.finish(true) end)
   end
 
-  return controller --[[@as SelectController]]
+  return controller --[[@as u.examples.SelectController]]
 end -- }}}
 
 --------------------------------------------------------------------------------
diff --git a/lua/u/buffer.lua b/lua/u/buffer.lua
index 0897a4f..0305349 100644
--- a/lua/u/buffer.lua
+++ b/lua/u/buffer.lua
@@ -5,7 +5,7 @@ local Renderer = require('u.renderer').Renderer
 --- @field bufnr number
 --- @field b vim.var_accessor
 --- @field bo vim.bo
---- @field private renderer u.Renderer
+--- @field renderer u.renderer.Renderer
 local Buffer = {}
 Buffer.__index = Buffer
 
@@ -75,6 +75,9 @@ function Buffer:autocmd(event, opts)
   vim.api.nvim_create_autocmd(event, vim.tbl_extend('force', opts, { buffer = self.bufnr }))
 end
 
+--- @param fn function
+function Buffer:call(fn) return vim.api.nvim_buf_call(self.bufnr, fn) end
+
 --- @param tree u.renderer.Tree
 function Buffer:render(tree) return self.renderer:render(tree) end
 
diff --git a/lua/u/codewriter.lua b/lua/u/codewriter.lua
index d8e810b..8dfb2b7 100644
--- a/lua/u/codewriter.lua
+++ b/lua/u/codewriter.lua
@@ -32,7 +32,7 @@ end
 --- @param line string
 --- @param bufnr? number
 function CodeWriter.from_line(line, bufnr)
-  if bufnr == nil then bufnr = vim.api.nvim_get_current_buf() end
+  if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
 
   local ws = line:match '^%s*'
   local expandtab = vim.api.nvim_get_option_value('expandtab', { buf = bufnr })
diff --git a/lua/u/logger.lua b/lua/u/logger.lua
index 07fdf04..f89480a 100644
--- a/lua/u/logger.lua
+++ b/lua/u/logger.lua
@@ -1,9 +1,9 @@
 local M = {}
 
+local LOG_ROOT = vim.fs.joinpath(vim.fn.stdpath 'cache', 'u.log')
+
 --- @params name string
-function M.file_for_name(name)
-  return vim.fs.joinpath(vim.fn.stdpath 'cache', 'u.log', name .. '.log.jsonl')
-end
+function M.file_for_name(name) return vim.fs.joinpath(LOG_ROOT, name .. '.log.jsonl') end
 
 --------------------------------------------------------------------------------
 -- Logger class
@@ -11,7 +11,6 @@ end
 
 --- @class u.Logger
 --- @field name string
---- @field private fd number
 local Logger = {}
 Logger.__index = Logger
 M.Logger = Logger
@@ -20,10 +19,7 @@ M.Logger = Logger
 function Logger.new(name)
   local file_path = M.file_for_name(name)
   vim.fn.mkdir(vim.fs.dirname(file_path), 'p')
-  local self = setmetatable({
-    name = name,
-    fd = (vim.uv or vim.loop).fs_open(file_path, 'a', tonumber('644', 8)),
-  }, Logger)
+  local self = setmetatable({ name = name }, Logger)
   return self
 end
 
@@ -32,10 +28,12 @@ end
 function Logger:write(level, ...)
   local data = { ... }
   if #data == 1 then data = data[1] end
-  (vim.uv or vim.loop).fs_write(
-    self.fd,
-    vim.json.encode { ts = os.date(), level = level, data = data } .. '\n'
+  local f = assert(io.open(M.file_for_name(self.name), 'a'), 'could not open file')
+  assert(
+    f:write(vim.json.encode { ts = os.date(), level = level, data = data } .. '\n'),
+    'could not write to file'
   )
+  f:close()
 end
 
 function Logger:trace(...) self:write('INFO', ...) end
@@ -64,6 +62,12 @@ function M.setup()
     vim.cmd.terminal('tail -f "' .. log_file_path .. '"')
     vim.cmd.startinsert()
   end, { nargs = '*' })
+
+  vim.api.nvim_create_user_command(
+    'Logroot',
+    function() vim.api.nvim_echo({ { LOG_ROOT } }, false, {}) end,
+    {}
+  )
 end
 
 return M
diff --git a/lua/u/pos.lua b/lua/u/pos.lua
index a3216d7..7b2b42b 100644
--- a/lua/u/pos.lua
+++ b/lua/u/pos.lua
@@ -67,6 +67,15 @@ function Pos.__sub(x, y)
   return x:next(-y)
 end
 
+--- @param bufnr number
+--- @param lnum number
+function Pos.from_eol(bufnr, lnum)
+  if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
+  local pos = Pos.new(bufnr, lnum, 0)
+  pos.col = pos:line():len()
+  return pos
+end
+
 --- @param name string
 --- @return u.Pos
 function Pos.from_pos(name)
@@ -96,6 +105,8 @@ end
 
 function Pos:as_vim() return { self.bufnr, self.lnum, self.col, self.off } end
 
+function Pos:eol() return Pos.from_eol(self.bufnr, self.lnum) end
+
 --- @param pos string
 function Pos:save_to_pos(pos) vim.fn.setpos(pos, { self.bufnr, self.lnum, self.col, self.off }) end
 
diff --git a/lua/u/range.lua b/lua/u/range.lua
index b8d8c0f..fbb0969 100644
--- a/lua/u/range.lua
+++ b/lua/u/range.lua
@@ -1,6 +1,13 @@
 local Pos = require 'u.pos'
 
 local ESC = vim.api.nvim_replace_termcodes('<Esc>', true, false, true)
+local NS = vim.api.nvim_create_namespace 'u.range'
+
+---@class u.ExtmarkRange
+---@field bufnr number
+---@field id number
+local ExtmarkRange = {}
+ExtmarkRange.__index = ExtmarkRange
 
 --- @class u.Range
 --- @field start u.Pos
@@ -83,6 +90,34 @@ function Range.from_marks(lpos, rpos)
   return Range.new(start, stop, mode)
 end
 
+--- @param bufnr number
+--- @param extmark vim.api.keyset.get_extmark_item_by_id
+function Range.from_extmark(bufnr, extmark)
+  ---@type integer, integer, vim.api.keyset.extmark_details | nil
+  local start_row0, start_col0, details = unpack(extmark)
+
+  local start = Pos.new(bufnr, start_row0 + 1, start_col0 + 1)
+  local stop = details and Pos.new(bufnr, details.end_row + 1, details.end_col)
+
+  if stop ~= nil then
+    -- Check for invalid extmark range:
+    if stop < start then return Range.new(stop) end
+
+    -- Check for stop-mark past the end of the buffer:
+    local buf_max_lines = vim.api.nvim_buf_line_count(bufnr)
+    if stop.lnum > buf_max_lines then
+      stop.lnum = buf_max_lines
+      stop = stop:eol()
+    end
+
+    -- A stop mark at position 0 means it is at the end of the last line.
+    -- Move it back.
+    if stop.col == 0 then stop = stop:must_next(-1) end
+  end
+
+  return Range.new(start, stop, 'v')
+end
+
 --- @param bufnr? number
 function Range.from_buf_text(bufnr)
   if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
@@ -115,7 +150,7 @@ end
 function Range.from_motion(motion, opts)
   -- Options handling:
   opts = opts or {}
-  if opts.bufnr == nil then opts.bufnr = vim.api.nvim_get_current_buf() end
+  if opts.bufnr == nil or opts.bufnr == 0 then opts.bufnr = vim.api.nvim_get_current_buf() end
   if opts.contains_cursor == nil then opts.contains_cursor = false end
   if opts.user_defined == nil then opts.user_defined = false end
 
@@ -146,6 +181,8 @@ function Range.from_motion(motion, opts)
     _G.Range__from_motion_opfunc = function(ty)
       _G.Range__from_motion_opfunc_captured_range = Range.from_op_func(ty)
     end
+    local old_eventignore = vim.o.eventignore
+    vim.o.eventignore = 'all'
     vim.go.operatorfunc = 'v:lua.Range__from_motion_opfunc'
     vim.cmd {
       cmd = 'normal',
@@ -153,6 +190,7 @@ function Range.from_motion(motion, opts)
       args = { ESC .. 'g@' .. motion },
       mods = { silent = true },
     }
+    vim.o.eventignore = old_eventignore
   end)
   local captured_range = _G.Range__from_motion_opfunc_captured_range
 
@@ -193,6 +231,26 @@ function Range.from_motion(motion, opts)
   return captured_range
 end
 
+--- @param opts? { contains_cursor?: boolean }
+function Range.from_tsquery_caps(bufnr, query, opts)
+  opts = opts or { contains_cursor = true }
+
+  local ranges = Range.from_buf_text(bufnr):tsquery(query)
+  if not ranges then return end
+  if not opts.contains_cursor then return ranges end
+
+  local cursor = Pos.from_pos '.'
+  return vim.tbl_map(function(cap_ranges)
+    return vim
+      .iter(cap_ranges)
+      :filter(
+        --- @param r u.Range
+        function(r) return r:contains(cursor) end
+      )
+      :totable()
+  end, ranges)
+end
+
 --- Get range information from the currently selected visual text.
 --- Note: from within a command mapping or an opfunc, use other specialized
 --- utilities, such as:
@@ -220,19 +278,22 @@ end
 --- @param args unknown
 --- @return u.Range|nil
 function Range.from_cmd_args(args)
-  --- @type 'v'|'V'
-  local mode
-  --- @type nil|u.Pos
-  local start
-  local stop
-  if args.range == 0 then
-    return nil
-  else
-    start = Pos.from_pos "'<"
-    stop = Pos.from_pos "'>"
-    mode = stop:is_col_max() and 'V' or 'v'
+  if args.range == 0 then return nil end
+
+  local bufnr = vim.api.nvim_get_current_buf()
+  if args.range == 1 then
+    return Range.new(Pos.new(bufnr, args.line1, 1), Pos.new(bufnr, args.line1, Pos.MAX_COL), 'V')
+  end
+
+  local is_visual = vim.fn.histget('cmd', -1):sub(1, 5) == [['<,'>]]
+  --- @type 'v'|'V'
+  local mode = is_visual and vim.fn.visualmode() or 'V'
+
+  if is_visual then
+    return Range.new(Pos.from_pos "'<", Pos.from_pos "'>", mode)
+  else
+    return Range.new(Pos.new(bufnr, args.line1, 1), Pos.new(bufnr, args.line2, Pos.MAX_COL), mode)
   end
-  return Range.new(start, stop, mode)
 end
 
 function Range.find_nearest_brackets()
@@ -272,6 +333,13 @@ function Range:to_linewise()
   return r
 end
 
+function Range:to_charwise()
+  local r = self:clone()
+  r.mode = 'v'
+  if r.stop:is_col_max() then r.stop = r.stop:as_real() end
+  return r
+end
+
 --- @param x u.Pos | u.Range
 function Range:contains(x)
   if getmetatable(x) == Pos then
@@ -313,25 +381,32 @@ end
 --- @param left string
 --- @param right string
 function Range:save_to_pos(left, right)
-  if self:is_empty() then
-    self.start:save_to_pos(left)
-    self.start:save_to_pos(right)
-  else
-    self.start:save_to_pos(left)
-    self.stop:save_to_pos(right)
-  end
+  self.start:save_to_pos(left);
+  (self:is_empty() and self.start or self.stop):save_to_pos(right)
 end
 
 --- @param left string
 --- @param right string
 function Range:save_to_marks(left, right)
-  if self:is_empty() then
-    self.start:save_to_mark(left)
-    self.start:save_to_mark(right)
-  else
-    self.start:save_to_mark(left)
-    self.stop:save_to_mark(right)
+  self.start:save_to_mark(left);
+  (self:is_empty() and self.start or self.stop):save_to_mark(right)
+end
+
+function Range:save_to_extmark()
+  local r = self:to_charwise()
+  local end_row = r.stop.lnum - 1
+  local end_col = r.stop.col
+  if self.mode == 'V' then
+    end_row = end_row + 1
+    end_col = 0
   end
+  local id = vim.api.nvim_buf_set_extmark(r.start.bufnr, NS, r.start.lnum - 1, r.start.col - 1, {
+    right_gravity = false,
+    end_right_gravity = true,
+    end_row = end_row,
+    end_col = end_col,
+  })
+  return ExtmarkRange.new(r.start.bufnr, id)
 end
 
 function Range:set_visual_selection()
@@ -348,14 +423,46 @@ function Range:set_visual_selection()
   self.stop:save_to_pos '.'
 end
 
---------------------------------------------------------------------------------
--- Range.from_* functions:
---------------------------------------------------------------------------------
-
 --------------------------------------------------------------------------------
 -- Text access/manipulation utilities:
 --------------------------------------------------------------------------------
 
+--- @param query string
+function Range:tsquery(query)
+  local bufnr = self.start.bufnr
+
+  local lang = vim.treesitter.language.get_lang(vim.bo[bufnr].filetype)
+  if lang == nil then return end
+  local parser = vim.treesitter.get_parser(bufnr, lang)
+  if parser == nil then return end
+  local tree = parser:parse()[1]
+  if tree == nil then return end
+
+  local root = tree:root()
+  local q = vim.treesitter.query.parse(lang, query)
+  --- @type table<string, u.Range[]>
+  local ranges = {}
+  for id, match, _meta in
+    q:iter_captures(root, bufnr, self.start.lnum - 1, (self.stop or self.start).lnum)
+  do
+    local start_row0, start_col0, stop_row0, stop_col0 = match:range()
+    local range = Range.new(
+      Pos.new(bufnr, start_row0 + 1, start_col0 + 1),
+      Pos.new(bufnr, stop_row0 + 1, stop_col0),
+      'v'
+    )
+    if range.stop.lnum > vim.api.nvim_buf_line_count(bufnr) then
+      range.stop = range.stop:must_next(-1)
+    end
+
+    local capture_name = q.captures[id]
+    if not ranges[capture_name] then ranges[capture_name] = {} end
+    if self:contains(range) then table.insert(ranges[capture_name], range) end
+  end
+
+  return ranges
+end
+
 function Range:length()
   if self:is_empty() then return 0 end
 
@@ -596,4 +703,17 @@ function Range:highlight(group, opts)
   }
 end
 
+function ExtmarkRange.new(bufnr, id) return setmetatable({ bufnr = bufnr, id = id }, ExtmarkRange) end
+
+function ExtmarkRange:range()
+  return Range.from_extmark(
+    self.bufnr,
+    vim.api.nvim_buf_get_extmark_by_id(self.bufnr, NS, self.id, {
+      details = true,
+    })
+  )
+end
+
+function ExtmarkRange:delete() vim.api.nvim_buf_del_extmark(self.bufnr, NS, self.id) end
+
 return Range
diff --git a/lua/u/renderer.lua b/lua/u/renderer.lua
index 1873ecc..aa68168 100644
--- a/lua/u/renderer.lua
+++ b/lua/u/renderer.lua
@@ -1,12 +1,23 @@
+function _G.URendererOpFuncSwallow() end
+
 local M = {}
 local H = {}
 
---- @alias u.renderer.Tag { kind: 'tag'; name: string, attributes: table<string, unknown>, children: u.renderer.Tree }
+--- @alias u.renderer.TagEventHandler fun(tag: u.renderer.Tag, mode: string, lhs: string): string
+
+--- @alias u.renderer.TagAttributes { [string]?: unknown; imap?: table<string, u.renderer.TagEventHandler>; nmap?: table<string, u.renderer.TagEventHandler>; vmap?: table<string, u.renderer.TagEventHandler>; xmap?: table<string, u.renderer.TagEventHandler>; omap?: table<string, u.renderer.TagEventHandler>, on_change?: fun(text: string): unknown }
+
+--- @class u.renderer.Tag
+--- @field kind 'tag'
+--- @field name string
+--- @field attributes u.renderer.TagAttributes
+--- @field children u.renderer.Tree
+
 --- @alias u.renderer.Node nil | boolean | string | u.renderer.Tag
 --- @alias u.renderer.Tree u.renderer.Node | u.renderer.Node[]
 
 -- luacheck: ignore
---- @type table<string, fun(attributes: table<string, any>, children: u.renderer.Tree): u.renderer.Tag> & fun(name: string, attributes: table<string, any>, children: u.renderer.Tree): u.renderer.Tag>
+--- @type table<string, fun(attributes: u.renderer.TagAttributes, children: u.renderer.Tree): u.renderer.Tag> & fun(name: string, attributes: u.renderer.TagAttributes, children: u.renderer.Tree): u.renderer.Tag>
 M.h = setmetatable({}, {
   __call = function(_, name, attributes, children)
     return {
@@ -17,7 +28,6 @@ M.h = setmetatable({}, {
     }
   end,
   __index = function(_, name)
-    -- vim.print('dynamic hl ' .. name)
     return function(attributes, children)
       return M.h('text', vim.tbl_deep_extend('force', { hl = name }, attributes), children)
     end
@@ -25,14 +35,19 @@ M.h = setmetatable({}, {
 })
 
 -- Renderer {{{
---- @alias RendererExtmark { id?: number; start: [number, number]; stop: [number, number]; opts: any; tag: any }
+--- @class u.renderer.RendererExtmark
+--- @field id? number
+--- @field start [number, number]
+--- @field stop [number, number]
+--- @field opts vim.api.keyset.set_extmark
+--- @field tag u.renderer.Tag
 
---- @class u.Renderer
+--- @class u.renderer.Renderer
 --- @field bufnr number
 --- @field ns number
 --- @field changedtick number
---- @field old { lines: string[]; extmarks: RendererExtmark[] }
---- @field curr { lines: string[]; extmarks: RendererExtmark[] }
+--- @field old { lines: string[]; extmarks: u.renderer.RendererExtmark[] }
+--- @field curr { lines: string[]; extmarks: u.renderer.RendererExtmark[] }
 local Renderer = {}
 Renderer.__index = Renderer
 M.Renderer = Renderer
@@ -51,7 +66,7 @@ function Renderer.is_tag_arr(x)
 end
 --- @param bufnr number|nil
 function Renderer.new(bufnr) -- {{{
-  if bufnr == nil then bufnr = vim.api.nvim_get_current_buf() end
+  if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
 
   if vim.b[bufnr]._renderer_ns == nil then
     vim.b[bufnr]._renderer_ns = vim.api.nvim_create_namespace('my.renderer:' .. tostring(bufnr))
@@ -64,6 +79,12 @@ function Renderer.new(bufnr) -- {{{
     old = { lines = {}, extmarks = {} },
     curr = { lines = {}, extmarks = {} },
   }, Renderer)
+
+  vim.api.nvim_create_autocmd({ 'TextChanged', 'TextChangedI', 'TextChangedP' }, {
+    buffer = bufnr,
+    callback = function() self:_on_text_changed() end,
+  })
+
   return self
 end -- }}}
 
@@ -128,7 +149,7 @@ function Renderer.markup_to_lines(opts) -- {{{
 end -- }}}
 
 --- @param opts {
----   tree: string;
+---   tree: u.renderer.Tree;
 ---   format_tag?: fun(tag: u.renderer.Tag): string;
 --- }
 function Renderer.markup_to_string(opts) return table.concat(Renderer.markup_to_lines(opts), '\n') end
@@ -136,7 +157,7 @@ function Renderer.markup_to_string(opts) return table.concat(Renderer.markup_to_
 --- @param bufnr number
 --- @param old_lines string[] | nil
 --- @param new_lines string[]
-function Renderer.patch_lines(bufnr, old_lines, new_lines)
+function Renderer.patch_lines(bufnr, old_lines, new_lines) -- {{{
   --
   -- Helpers:
   --
@@ -189,7 +210,7 @@ function Renderer.patch_lines(bufnr, old_lines, new_lines)
       -- No change
     end
   end
-end
+end -- }}}
 
 --- @param tree u.renderer.Tree
 function Renderer:render(tree) -- {{{
@@ -199,7 +220,7 @@ function Renderer:render(tree) -- {{{
     self.changedtick = changedtick
   end
 
-  --- @type RendererExtmark[]
+  --- @type u.renderer.RendererExtmark[]
   local extmarks = {}
 
   --- @type string[]
@@ -214,31 +235,36 @@ function Renderer:render(tree) -- {{{
           tag.attributes.extmark.hl_group = tag.attributes.extmark.hl_group or hl
         end
 
-        local extmark = tag.attributes.extmark
+        local extmark_opts = tag.attributes.extmark or {}
 
         -- Set any necessary keymaps:
         for _, mode in ipairs { 'i', 'n', 'v', 'x', 'o' } do
           for lhs, _ in pairs(tag.attributes[mode .. 'map'] or {}) do
             -- Force creating an extmark if there are key handlers. To accurately
             -- sense the bounds of the text, we need an extmark:
-            extmark = extmark or {}
-            vim.keymap.set(
-              'n',
-              lhs,
-              function() return self:_expr_map_callback('n', lhs) end,
-              { buffer = self.bufnr, expr = true, replace_keycodes = true }
-            )
+            vim.keymap.set(mode, lhs, function()
+              local result = self:_expr_map_callback(mode, lhs)
+              -- If the handler indicates that it wants to swallow the event,
+              -- we have to convert that intention into something compatible
+              -- with expr-mappings, which don't support '<Nop>' (they try to
+              -- execute the literal characters). We'll use the 'g@' operator
+              -- to do that, forwarding the event to an operatorfunc that does
+              -- nothing:
+              if result == '' then
+                vim.go.operatorfunc = 'v:lua.URendererOpFuncSwallow'
+                return 'g@ '
+              end
+              return result
+            end, { buffer = self.bufnr, expr = true, replace_keycodes = true })
           end
         end
 
-        if extmark then
-          table.insert(extmarks, {
-            start = start0,
-            stop = stop0,
-            opts = extmark,
-            tag = tag,
-          })
-        end
+        table.insert(extmarks, {
+          start = start0,
+          stop = stop0,
+          opts = extmark_opts,
+          tag = tag,
+        })
       end
     end, -- }}}
   }
@@ -248,25 +274,6 @@ function Renderer:render(tree) -- {{{
   self:_reconcile()
 end -- }}}
 
---- @private
---- @param start integer
---- @param end_ integer
---- @param strict_indexing boolean
---- @param replacement string[]
-function Renderer:_set_lines(start, end_, strict_indexing, replacement)
-  vim.api.nvim_buf_set_lines(self.bufnr, start, end_, strict_indexing, replacement)
-end
-
---- @private
---- @param start_row integer
---- @param start_col integer
---- @param end_row integer
---- @param end_col integer
---- @param replacement string[]
-function Renderer:_set_text(start_row, start_col, end_row, end_col, replacement)
-  vim.api.nvim_buf_set_text(self.bufnr, start_row, start_col, end_row, end_col, replacement)
-end
-
 --- @private
 function Renderer:_reconcile() -- {{{
   --
@@ -277,10 +284,15 @@ function Renderer:_reconcile() -- {{{
 
   --
   -- Step 2: reconcile extmarks:
+  -- You may be tempted to try to keep track of which extmarks are needed, and
+  -- only delete those that are not needed. However, each time a tree is
+  -- rendered, brand new extmarks are created. For simplicity, it is better to
+  -- just delete all extmarks, and recreate them.
   --
 
   -- Clear current extmarks:
   vim.api.nvim_buf_clear_namespace(self.bufnr, self.ns, 0, -1)
+
   -- Set current extmarks:
   for _, extmark in ipairs(self.curr.extmarks) do
     extmark.id = vim.api.nvim_buf_set_extmark(
@@ -292,6 +304,13 @@ function Renderer:_reconcile() -- {{{
         id = extmark.id,
         end_row = extmark.stop[1],
         end_col = extmark.stop[2],
+        -- If we change the text starting from the beginning (where the extmark
+        -- is), we don't want the extmark to move to the right.
+        right_gravity = false,
+        -- If we change the text starting from the end (where the end extmark
+        -- is), we don't want the extmark to stay stationary: we want it to
+        -- move to the right.
+        end_right_gravity = true,
       }, extmark.opts)
     )
   end
@@ -306,7 +325,7 @@ function Renderer:_expr_map_callback(mode, lhs) -- {{{
   -- find the tag with the smallest intersection that contains the cursor:
   local pos0 = vim.api.nvim_win_get_cursor(0)
   pos0[1] = pos0[1] - 1 -- make it actually 0-based
-  local pos_infos = self:get_pos_infos(pos0)
+  local pos_infos = self:get_tags_at(pos0)
 
   if #pos_infos == 0 then return lhs end
 
@@ -316,9 +335,10 @@ function Renderer:_expr_map_callback(mode, lhs) -- {{{
     local tag = pos_info.tag
 
     -- is the tag listening?
+    --- @type u.renderer.TagEventHandler?
     local f = vim.tbl_get(tag.attributes, mode .. 'map', lhs)
     if type(f) == 'function' then
-      local result = f()
+      local result = f(tag, mode, lhs)
       if result == '' then
         -- bubble-up to the next tag, but set cancel to true, in case there are
         -- no more tags to bubble up to:
@@ -333,21 +353,145 @@ function Renderer:_expr_map_callback(mode, lhs) -- {{{
   return cancel and '' or lhs
 end -- }}}
 
+function Renderer:_on_text_changed() -- {{{
+  -- Reset changedtick, so that the reconciler knows to refresh its cached
+  -- buffer-content before computing the diff:
+  self.changedtick = 0
+
+  --- @type integer, integer
+  local l, c = unpack(vim.api.nvim_win_get_cursor(0))
+  l = l - 1 -- make it actually 0-based
+  local pos_infos = self:get_tags_at({ l, c }, 'i')
+  for _, pos_info in ipairs(pos_infos) do
+    local extmark_inf = pos_info.extmark
+    local tag = pos_info.tag
+
+    local on_change = tag.attributes.on_change
+    if on_change and type(on_change) == 'function' then
+      local extmark =
+        vim.api.nvim_buf_get_extmark_by_id(self.bufnr, self.ns, extmark_inf.id, { details = true })
+
+      --- @type integer, integer, vim.api.keyset.extmark_details
+      local start_row0, start_col0, details = unpack(extmark)
+      local end_row0, end_col0 = details.end_row, details.end_col
+
+      local buf_max_line0 = math.max(1, vim.api.nvim_buf_line_count(self.bufnr) - 1)
+      if end_row0 > buf_max_line0 then
+        end_row0 = buf_max_line0
+        local last_line = vim.api.nvim_buf_get_lines(self.bufnr, end_row0, end_row0 + 1, false)[1]
+          or ''
+        end_col0 = last_line:len()
+      end
+      if end_col0 == 0 then
+        end_row0 = end_row0 - 1
+        local last_line = vim.api.nvim_buf_get_lines(self.bufnr, end_row0, end_row0 + 1, false)[1]
+          or ''
+        end_col0 = last_line:len()
+      end
+
+      if start_row0 == end_row0 and start_col0 == end_col0 then
+        on_change ''
+      else
+        local pos1 = { self.bufnr, start_row0 + 1, start_col0 + 1 }
+        local pos2 = { self.bufnr, end_row0 + 1, end_col0 }
+        local ok, lines = pcall(vim.fn.getregion, pos1, pos2, { type = 'v' })
+        if not ok then
+          vim.api.nvim_echo({
+            { '(u.nvim:getregion:invalid-pos) ', 'ErrorMsg' },
+            {
+              '{ start, end } = ' .. vim.inspect({ pos1, pos2 }, { newline = ' ', indent = '' }),
+            },
+          }, true, {})
+          error(lines)
+        end
+        local text = table.concat(lines, '\n')
+        on_change(text)
+      end
+    end
+  end
+end -- }}}
+
+--- @private
+function Renderer:_debug() -- {{{
+  local prev_w = vim.api.nvim_get_current_win()
+  vim.cmd.vnew()
+  local info_bufnr = vim.api.nvim_get_current_buf()
+  vim.bo.bufhidden = 'delete'
+  vim.bo.buflisted = false
+  vim.bo.buftype = 'nowrite'
+
+  local ids = {}
+  local function cleanup()
+    for _, id in ipairs(ids) do
+      vim.api.nvim_del_autocmd(id)
+    end
+    vim.api.nvim_buf_delete(info_bufnr, { force = true })
+  end
+
+  table.insert(
+    ids,
+    vim.api.nvim_create_autocmd({ 'CursorMoved', 'CursorMovedI' }, {
+      callback = function()
+        if vim.api.nvim_get_current_win() ~= prev_w then return end
+
+        local l, c = unpack(vim.api.nvim_win_get_cursor(0))
+        l = l - 1 -- make it actually 0-based
+
+        local info = {
+          cursor = {
+            pos = { l, c },
+            tags = self:get_tags_at { l, c },
+            extmarks = vim.api.nvim_buf_get_extmarks(
+              self.bufnr,
+              self.ns,
+              { l, c },
+              { l, c },
+              { details = true, overlap = true }
+            ),
+          },
+          computed = {
+            extmarks = self.curr.extmarks,
+          },
+        }
+        vim.api.nvim_buf_set_lines(info_bufnr, 0, -1, true, vim.split(vim.inspect(info), '\n'))
+      end,
+    })
+  )
+  table.insert(
+    ids,
+    vim.api.nvim_create_autocmd('WinClosed', {
+      pattern = tostring(vim.api.nvim_get_current_win()),
+      callback = cleanup,
+    })
+  )
+  table.insert(
+    ids,
+    vim.api.nvim_create_autocmd('WinClosed', {
+      pattern = tostring(prev_w),
+      callback = cleanup,
+    })
+  )
+
+  vim.api.nvim_set_current_win(prev_w)
+end -- }}}
+
 --- Returns pairs of extmarks and tags associate with said extmarks. The
 --- returned tags/extmarks are sorted smallest (innermost) to largest
 --- (outermost).
 ---
 --- @private (private for now)
 --- @param pos0 [number; number]
---- @return { extmark: RendererExtmark; tag: u.renderer.Tag; }[]
-function Renderer:get_pos_infos(pos0) -- {{{
+--- @param mode string?
+--- @return { extmark: u.renderer.RendererExtmark; tag: u.renderer.Tag; }[]
+function Renderer:get_tags_at(pos0, mode) -- {{{
   local cursor_line0, cursor_col0 = pos0[1], pos0[2]
+  if not mode then mode = vim.api.nvim_get_mode().mode end
 
   -- The cursor (block) occupies **two** extmark spaces: one for it's left
   -- edge, and one for it's right. We need to do our own intersection test,
   -- because the NeoVim API is over-inclusive in what it returns:
-  --- @type RendererExtmark[]
-  local intersecting_extmarks = vim
+  --- @type u.renderer.RendererExtmark[]
+  local mapped_extmarks = vim
     .iter(
       vim.api.nvim_buf_get_extmarks(
         self.bufnr,
@@ -357,7 +501,7 @@ function Renderer:get_pos_infos(pos0) -- {{{
         { details = true, overlap = true }
       )
     )
-    --- @return RendererExtmark
+    --- @return u.renderer.RendererExtmark
     :map(function(ext)
       --- @type number, number, number, { end_row?: number; end_col?: number }|nil
       local id, line0, col0, details = unpack(ext)
@@ -368,13 +512,43 @@ function Renderer:get_pos_infos(pos0) -- {{{
       end
       return { id = id, start = start, stop = stop, opts = details }
     end)
-    --- @param ext RendererExtmark
+    :totable()
+
+  local intersecting_extmarks = vim
+    .iter(mapped_extmarks)
+    --- @param ext u.renderer.RendererExtmark
     :filter(function(ext)
       if ext.stop[1] ~= nil and ext.stop[2] ~= nil then
-        return cursor_line0 >= ext.start[1]
-          and cursor_col0 >= ext.start[2]
-          and cursor_line0 <= ext.stop[1]
-          and cursor_col0 < ext.stop[2]
+        -- If we've "ciw" and "collapsed" an extmark onto the cursor,
+        -- the cursor pos will equal the exmark's start AND end. In this
+        -- case, we want to include the extmark.
+        if
+          cursor_line0 == ext.start[1]
+          and cursor_col0 == ext.start[2]
+          and cursor_line0 == ext.stop[1]
+          and cursor_col0 == ext.stop[2]
+        then
+          return true
+        end
+
+        return
+          -- START: line check
+          cursor_line0 >= ext.start[1]
+            -- START: column check
+            and (cursor_line0 ~= ext.start[1] or cursor_col0 >= ext.start[2])
+            -- STOP: line check
+            and cursor_line0 <= ext.stop[1]
+            -- STOP: column check
+            and (
+              cursor_line0 ~= ext.stop[1]
+              or (
+                mode == 'i'
+                  -- In insert mode, the cursor is "thin", so <= to compensate:
+                  and cursor_col0 <= ext.stop[2]
+                -- In normal mode, the cursor is "wide", so < to compensate:
+                or cursor_col0 < ext.stop[2]
+              )
+            )
       else
         return true
       end
@@ -384,8 +558,8 @@ function Renderer:get_pos_infos(pos0) -- {{{
   -- Sort the tags into smallest (inner) to largest (outer):
   table.sort(
     intersecting_extmarks,
-    --- @param x1 RendererExtmark
-    --- @param x2 RendererExtmark
+    --- @param x1 u.renderer.RendererExtmark
+    --- @param x2 u.renderer.RendererExtmark
     function(x1, x2)
       if
         x1.start[1] == x2.start[1]
@@ -407,10 +581,10 @@ function Renderer:get_pos_infos(pos0) -- {{{
   -- created extmarks in self.curr.extmarks, which also has which tag each
   -- extmark is associated with. Cross-reference with that list to get a list
   -- of tags that we need to fire events for:
-  --- @type { extmark: RendererExtmark; tag: u.renderer.Tag }[]
+  --- @type { extmark: u.renderer.RendererExtmark; tag: u.renderer.Tag }[]
   local matching_tags = vim
     .iter(intersecting_extmarks)
-    --- @param ext RendererExtmark
+    --- @param ext u.renderer.RendererExtmark
     :map(function(ext)
       for _, extmark_cache in ipairs(self.curr.extmarks) do
         if extmark_cache.id == ext.id then return { extmark = ext, tag = extmark_cache.tag } end
@@ -420,10 +594,23 @@ function Renderer:get_pos_infos(pos0) -- {{{
 
   return matching_tags
 end -- }}}
+
+--- @private
+--- @param tag_or_id string | u.renderer.Tag
+--- @return { start: [number, number]; stop: [number, number] } | nil
+function Renderer:get_tag_bounds(tag_or_id) -- {{{
+  for _, x in ipairs(self.curr.extmarks) do
+    local pos = { start = x.start, stop = x.stop }
+    local does_tag_match = type(tag_or_id) == 'string' and x.tag.attributes.id == tag_or_id
+      or x.tag == tag_or_id
+    if does_tag_match then return pos end
+  end
+end -- }}}
+
 -- }}}
 
 -- TreeBuilder {{{
---- @class u.TreeBuilder
+--- @class u.renderer.TreeBuilder
 --- @field private nodes u.renderer.Node[]
 local TreeBuilder = {}
 TreeBuilder.__index = TreeBuilder
@@ -435,7 +622,7 @@ function TreeBuilder.new()
 end
 
 --- @param nodes u.renderer.Tree
---- @return u.TreeBuilder
+--- @return u.renderer.TreeBuilder
 function TreeBuilder:put(nodes)
   table.insert(self.nodes, nodes)
   return self
@@ -444,15 +631,15 @@ end
 --- @param name string
 --- @param attributes? table<string, any>
 --- @param children? u.renderer.Node | u.renderer.Node[]
---- @return u.TreeBuilder
+--- @return u.renderer.TreeBuilder
 function TreeBuilder:put_h(name, attributes, children)
   local tag = M.h(name, attributes, children)
   table.insert(self.nodes, tag)
   return self
 end
 
---- @param fn fun(TreeBuilder): any
---- @return u.TreeBuilder
+--- @param fn fun(tb: u.renderer.TreeBuilder): any
+--- @return u.renderer.TreeBuilder
 function TreeBuilder:nest(fn)
   local nested_writer = TreeBuilder.new()
   fn(nested_writer)
@@ -460,19 +647,40 @@ function TreeBuilder:nest(fn)
   return self
 end
 
+--- @generic T
+--- @param arr T[]
+--- @param f fun(tb: u.renderer.TreeBuilder, item: T, idx: number): any
+function TreeBuilder:ipairs(arr, f)
+  return self:nest(function(tb)
+    for idx, item in ipairs(arr) do
+      f(tb, item, idx)
+    end
+  end)
+end
+
+--- @param tab table
+--- @param f fun(tb: u.renderer.TreeBuilder, key: any, value: any): any
+function TreeBuilder:pairs(tab, f)
+  return self:nest(function(tb)
+    for k, v in pairs(tab) do
+      f(tb, k, v)
+    end
+  end)
+end
+
 --- @return u.renderer.Tree
 function TreeBuilder:tree() return self.nodes end
 -- }}}
 
 -- Levenshtein utility {{{
 -- luacheck: ignore
---- @alias LevenshteinChange<T> ({ kind: 'add'; item: T; index: number; } | { kind: 'delete'; item: T; index: number; } | { kind: 'change'; from: T; to: T; index: number; })
+--- @alias u.renderer.LevenshteinChange<T> ({ kind: 'add'; item: T; index: number; } | { kind: 'delete'; item: T; index: number; } | { kind: 'change'; from: T; to: T; index: number; })
 --- @private
 --- @generic T
 --- @param x `T`[]
 --- @param y T[]
 --- @param cost? { of_delete?: fun(x: T): number; of_add?: fun(x: T): number; of_change?: fun(x: T, y: T): number; }
---- @return LevenshteinChange<T>[] The changes, from last (greatest index) to first (smallest index).
+--- @return u.renderer.LevenshteinChange<T>[] The changes, from last (greatest index) to first (smallest index).
 function H.levenshtein(x, y, cost)
   -- At the moment, this whole `cost` plumbing is not used. Deletes have the
   -- same cost as Adds or Changes. I can imagine a future, however, where
@@ -523,7 +731,7 @@ function H.levenshtein(x, y, cost)
   -- Backtrack to find the changes
   local i = m
   local j = n
-  --- @type LevenshteinChange[]
+  --- @type u.renderer.LevenshteinChange[]
   local changes = {}
 
   while i > 0 or j > 0 do
@@ -540,7 +748,7 @@ function H.levenshtein(x, y, cost)
     if is_first_min(cost_of_change, cost_of_add, cost_of_delete) then
       -- potential change
       if x[i] ~= y[j] then
-        --- @type LevenshteinChange
+        --- @type u.renderer.LevenshteinChange
         local change = { kind = 'change', from = x[i], index = i, to = y[j] }
         table.insert(changes, change)
       end
@@ -548,13 +756,13 @@ function H.levenshtein(x, y, cost)
       j = j - 1
     elseif is_first_min(cost_of_add, cost_of_change, cost_of_delete) then
       -- addition
-      --- @type LevenshteinChange
+      --- @type u.renderer.LevenshteinChange
       local change = { kind = 'add', item = y[j], index = i + 1 }
       table.insert(changes, change)
       j = j - 1
     elseif is_first_min(cost_of_delete, cost_of_change, cost_of_add) then
       -- deletion
-      --- @type LevenshteinChange
+      --- @type u.renderer.LevenshteinChange
       local change = { kind = 'delete', item = x[i], index = i }
       table.insert(changes, change)
       i = i - 1
diff --git a/lua/u/utils.lua b/lua/u/utils.lua
index d0d7d90..9427dc4 100644
--- a/lua/u/utils.lua
+++ b/lua/u/utils.lua
@@ -4,15 +4,63 @@ local M = {}
 -- Types
 --
 
---- @alias QfItem { col: number, filename: string, kind: string, lnum: number, text: string }
---- @alias KeyMaps table<string, fun(): any | string> }
--- luacheck: ignore
---- @alias CmdArgs { args: string; bang: boolean; count: number; fargs: string[]; line1: number; line2: number; mods: string; name: string; range: 0|1|2; reg: string; smods: any; info: u.Range|nil }
+--- @class u.utils.QfItem
+--- @field col number
+--- @field filename string
+--- @field kind string
+--- @field lnum number
+--- @field text string
 
+--- @class u.utils.RawCmdArgs
+--- @field args string
+--- @field bang boolean
+--- @field count number
+--- @field fargs string[]
+--- @field line1 number
+--- @field line2 number
+--- @field mods string
+--- @field name string
+--- @field range 0|1|2
+--- @field reg string
+--- @field smods any
+
+--- @class u.utils.CmdArgs: u.utils.RawCmdArgs
+--- @field info u.Range|nil
+
+--- @class u.utils.UcmdArgs
+--- @field nargs? 0|1|'*'|'?'|'+'
+--- @field range? boolean|'%'|number
+--- @field count? boolean|number
+--- @field addr? string
+--- @field completion? string
+--- @field force? boolean
+--- @field preview? fun(opts: u.utils.UcmdArgs, ns: integer, buf: integer):0|1|2
+
+--
+-- Functions
+--
+
+--- Debug utility that prints a value and returns it unchanged.
+--- Useful for debugging in the middle of expressions or function chains.
+---
 --- @generic T
---- @param x `T`
---- @param message? string
---- @return T
+--- @param x `T` The value to debug print
+--- @param message? string Optional message to print alongside the value
+--- @return T The original value, unchanged
+---
+--- @usage
+--- ```lua
+--- -- Debug a value in the middle of a chain:
+--- local result = some_function()
+---   :map(utils.dbg)  -- prints the intermediate value
+---   :filter(predicate)
+---
+--- -- Debug with a custom message:
+--- local config = utils.dbg(get_config(), "Current config:")
+---
+--- -- Debug return values:
+--- return utils.dbg(calculate_result(), "Final result")
+--- ```
 function M.dbg(x, message)
   local t = {}
   if message ~= nil then table.insert(t, message) end
@@ -21,22 +69,37 @@ function M.dbg(x, message)
   return x
 end
 
---- A utility for creating user commands that also pre-computes useful information
---- and attaches it to the arguments.
+--- Creates a user command with enhanced argument processing.
+--- Automatically computes range information and attaches it as `args.info`.
 ---
+--- @param name string The command name (without the leading colon)
+--- @param cmd string | fun(args: u.utils.CmdArgs): any Command implementation
+--- @param opts? u.utils.UcmdArgs Command options (nargs, range, etc.)
+---
+--- @usage
 --- ```lua
---- -- Example:
---- ucmd('MyCmd', function(args)
----   -- print the visually selected text:
+--- -- Create a command that works with visual selections:
+--- utils.ucmd('MyCmd', function(args)
+---   -- Print the visually selected text:
 ---   vim.print(args.info:text())
----   -- or get the vtext as an array of lines:
+---   -- Or get the selection as an array of lines:
 ---   vim.print(args.info:lines())
 --- end, { nargs = '*', range = true })
+---
+--- -- Create a command that processes the current line:
+--- utils.ucmd('ProcessLine', function(args)
+---   local line_text = args.info:text()
+---   -- Process the line...
+--- end, { range = '%' })
+---
+--- -- Create a command with arguments:
+--- utils.ucmd('SearchReplace', function(args)
+---   local pattern, replacement = args.fargs[1], args.fargs[2]
+---   local text = args.info:text()
+---   -- Perform search and replace...
+--- end, { nargs = 2, range = true })
 --- ```
---- @param name string
---- @param cmd string | fun(args: CmdArgs): any
 -- luacheck: ignore
---- @param opts? { nargs?: 0|1|'*'|'?'|'+'; range?: boolean|'%'|number; count?: boolean|number, addr?: string; completion?: string }
 function M.ucmd(name, cmd, opts)
   local Range = require 'u.range'
 
@@ -48,9 +111,81 @@ function M.ucmd(name, cmd, opts)
       return cmd(args)
     end
   end
-  vim.api.nvim_create_user_command(name, cmd2, opts or {})
+  vim.api.nvim_create_user_command(name, cmd2, opts or {} --[[@as any]])
 end
 
+--- Creates command arguments for delegating from one command to another.
+--- Preserves all relevant context (range, modifiers, bang, etc.) when
+--- implementing a derived command in terms of a base command.
+---
+--- @param current_args vim.api.keyset.create_user_command.command_args|u.utils.RawCmdArgs The arguments from the current command
+--- @return vim.api.keyset.cmd Arguments suitable for vim.cmd() calls
+---
+--- @usage
+--- ```lua
+--- -- Implement :MyEdit in terms of :edit, preserving all context:
+--- utils.ucmd('MyEdit', function(args)
+---   local delegated_args = utils.create_delegated_cmd_args(args)
+---   -- Add custom logic here...
+---   vim.cmd.edit(delegated_args)
+--- end, { nargs = '*', range = true, bang = true })
+---
+--- -- Implement :MySubstitute that delegates to :substitute:
+--- utils.ucmd('MySubstitute', function(args)
+---   -- Pre-process arguments
+---   local pattern = preprocess_pattern(args.fargs[1])
+---
+---   local delegated_args = utils.create_delegated_cmd_args(args)
+---   delegated_args.args = { pattern, args.fargs[2] }
+---
+---   vim.cmd.substitute(delegated_args)
+--- end, { nargs = 2, range = true, bang = true })
+--- ```
+function M.create_delegated_cmd_args(current_args)
+  --- @type vim.api.keyset.cmd
+  local args = {
+    range = current_args.range == 1 and { current_args.line1 }
+      or current_args.range == 2 and { current_args.line1, current_args.line2 }
+      or nil,
+    count = (current_args.count ~= -1 and current_args.range == 0) and current_args.count or nil,
+    reg = current_args.reg ~= '' and current_args.reg or nil,
+    bang = current_args.bang or nil,
+    args = #current_args.fargs > 0 and current_args.fargs or nil,
+    mods = current_args.smods,
+  }
+  return args
+end
+
+--- Gets the current editor dimensions.
+--- Useful for positioning floating windows or calculating layout sizes.
+---
+--- @return { width: number, height: number } The editor dimensions in columns and lines
+---
+--- @usage
+--- ```lua
+--- -- Center a floating window:
+--- local dims = utils.get_editor_dimensions()
+--- local win_width = 80
+--- local win_height = 20
+--- local col = math.floor((dims.width - win_width) / 2)
+--- local row = math.floor((dims.height - win_height) / 2)
+---
+--- vim.api.nvim_open_win(bufnr, true, {
+---   relative = 'editor',
+---   width = win_width,
+---   height = win_height,
+---   col = col,
+---   row = row,
+--- })
+---
+--- -- Check if editor is wide enough for side-by-side layout:
+--- local dims = utils.get_editor_dimensions()
+--- if dims.width >= 160 then
+---   -- Use side-by-side layout
+--- else
+---   -- Use stacked layout
+--- end
+--- ```
 function M.get_editor_dimensions() return { width = vim.go.columns, height = vim.go.lines } end
 
 return M
diff --git a/shell.nix b/shell.nix
index aecdea1..d172772 100644
--- a/shell.nix
+++ b/shell.nix
@@ -1,10 +1,10 @@
 {
   pkgs ?
     import
-      # nixpkgs-unstable (neovim@0.11.2):
+      # nixos-25.05 (neovim@0.11.2):
       (fetchTarball {
-        url = "https://github.com/nixos/nixpkgs/archive/e4b09e47ace7d87de083786b404bf232eb6c89d8.tar.gz";
-        sha256 = "1a2qvp2yz8j1jcggl1yvqmdxicbdqq58nv7hihmw3bzg9cjyqm26";
+        url = "https://github.com/nixos/nixpkgs/archive/32a4e87942101f1c9f9865e04dc3ddb175f5f32e.tar.gz";
+        sha256 = "1jvflnbrxa8gjxkwjq6kdpdzgwp0hs59h9l3xjasksv0v7xlwykz";
       })
       { },
 }:
@@ -19,5 +19,6 @@ pkgs.mkShell {
     pkgs.lua51Packages.nlua
     pkgs.neovim
     pkgs.stylua
+    pkgs.watchexec
   ];
 }
diff --git a/spec/range_spec.lua b/spec/range_spec.lua
index f30dd40..3c3079b 100644
--- a/spec/range_spec.lua
+++ b/spec/range_spec.lua
@@ -238,6 +238,128 @@ describe('Range', function()
     end)
   end)
 
+  it('from_tsquery_caps', function()
+    withbuf({
+      '-- a comment',
+      '',
+      'function foo(bar) end',
+      '',
+      '-- a middle comment',
+      '',
+      'function bar(baz) end',
+      '',
+      '-- another comment',
+    }, function()
+      vim.cmd.setfiletype 'lua'
+
+      --- @param contains_cursor? boolean
+      local function get_caps(contains_cursor)
+        if contains_cursor == nil then contains_cursor = true end
+        return (Range.from_tsquery_caps(
+          0,
+          '(function_declaration) @f',
+          { contains_cursor = contains_cursor }
+        )).f or {}
+      end
+
+      local caps = get_caps(false)
+      assert.are.same(#caps, 2)
+      assert.are.same(vim.iter(caps):map(function(c) return c:text() end):totable(), {
+        'function foo(bar) end',
+        'function bar(baz) end',
+      })
+
+      Pos.new(0, 1, 1):save_to_pos '.'
+      caps = get_caps()
+      assert.are.same(#caps, 0)
+
+      Pos.new(0, 3, 18):save_to_pos '.'
+      caps = get_caps()
+      assert.are.same(#caps, 1)
+      assert.are.same(caps[1]:text(), 'function foo(bar) end')
+
+      Pos.new(0, 5, 1):save_to_pos '.'
+      caps = get_caps()
+      assert.are.same(#caps, 0)
+
+      Pos.new(0, 7, 1):save_to_pos '.'
+      caps = get_caps()
+      assert.are.same(#caps, 1)
+      assert.are.same(caps[1]:text(), 'function bar(baz) end')
+    end)
+  end)
+
+  it('from_tsquery_caps with string array filter', function()
+    withbuf({
+      '{',
+      '  sample_key1 = "sample-value1",',
+      '  sample_key2 = "sample-value2",',
+      '}',
+    }, function()
+      vim.cmd.setfiletype 'lua'
+
+      -- Place cursor in "sample-value1"
+      Pos.new(0, 2, 25):save_to_pos '.'
+
+      -- Query that captures both keys and values in pairs
+      local query = [[
+        (field
+          name: _ @key
+          value: _ @value)
+      ]]
+
+      local ranges = Range.from_line(0, 2):tsquery(query)
+
+      -- Should have both @key and @value captures for the first pair only
+      -- (since cursor is in sample-value1)
+      assert(ranges, 'Range should not be nil')
+      assert(ranges.key, 'Range.key should not be nil')
+      assert(ranges.value, 'Range.value should not be nil')
+
+      -- Should have exactly one key and one value
+      assert.are.same(#ranges.key, 1)
+      assert.are.same(#ranges.value, 1)
+
+      -- Check that we got sample-key1 and sample-value1
+      assert.are.same(ranges.key[1]:text(), 'sample_key1')
+      assert.are.same(ranges.value[1]:text(), '"sample-value1"')
+    end)
+
+    -- Make sure this works when the match is on the last line:
+    withbuf({
+      '{sample_key1= "sample-value1",',
+      'sample_key2= "sample-value2"}',
+    }, function()
+      vim.cmd.setfiletype 'lua'
+
+      -- Place cursor in "sample-value1"
+      Pos.new(0, 2, 25):save_to_pos '.'
+
+      -- Query that captures both keys and values in pairs
+      local query = [[
+        (field
+          name: _ @key
+          value: _ @value)
+      ]]
+
+      local ranges = Range.from_line(0, 2):tsquery(query)
+
+      -- Should have both @key and @value captures for the first pair only
+      -- (since cursor is in sample-value1)
+      assert(ranges, 'Range should not be nil')
+      assert(ranges.key, 'Range.key should not be nil')
+      assert(ranges.value, 'Range.value should not be nil')
+
+      -- Should have exactly one key and one value
+      assert.are.same(#ranges.key, 1)
+      assert.are.same(#ranges.value, 1)
+
+      -- Check that we got sample-key2 and sample-value2
+      assert.are.same(ranges.key[1]:text(), 'sample_key2')
+      assert.are.same(ranges.value[1]:text(), '"sample-value2"')
+    end)
+  end)
+
   it('should get nearest block', function()
     withbuf({
       'this is a {',
@@ -318,18 +440,89 @@ describe('Range', function()
     end)
   end)
 
-  it('from_cmd_args', function()
-    local args = { range = 1 }
+  it('from_cmd_args: range=0', function()
+    local args = { range = 0 }
+    withbuf(
+      { 'line one', 'and line two' },
+      function() assert.are.same(Range.from_cmd_args(args), nil) end
+    )
+  end)
+
+  it('from_cmd_args: range=1', function()
+    local args = { range = 1, line1 = 1 }
     withbuf({ 'line one', 'and line two' }, function()
       local a = Pos.new(nil, 1, 1)
-      local b = Pos.new(nil, 2, 2)
+      local b = Pos.new(nil, 1, Pos.MAX_COL)
+
+      local range = Range.from_cmd_args(args) --[[@as u.Range]]
+      assert.are.same(range.start, a)
+      assert.are.same(range.stop, b)
+      assert.are.same(range.mode, 'V')
+      assert.are.same(range:text(), 'line one')
+    end)
+  end)
+
+  it('from_cmd_args: range=2: no-visual', function()
+    local args = { range = 2, line1 = 1, line2 = 2 }
+    withbuf({ 'line one', 'and line two' }, function()
+      local range = Range.from_cmd_args(args) --[[@as u.Range]]
+      assert.are.same(range.start, Pos.new(nil, 1, 1))
+      assert.are.same(range.stop, Pos.new(nil, 2, Pos.MAX_COL))
+      assert.are.same(range.mode, 'V')
+      assert.are.same(range:text(), 'line one\nand line two')
+    end)
+  end)
+
+  it('from_cmd_args: range=2: visual: linewise', function()
+    local args = { range = 2, line1 = 1, line2 = 2 }
+    withbuf({ 'line one', 'and line two' }, function()
+      local a = Pos.new(nil, 1, 1)
+      local b = Pos.new(nil, 2, Pos.MAX_COL)
       a:save_to_pos "'<"
       b:save_to_pos "'>"
+      local range = Range.from_cmd_args(args) --[[@as u.Range]]
+      assert.are.same(range.start, a)
+      assert.are.same(range.stop, b)
+      assert.are.same(range.mode, 'V')
+      assert.are.same(range:text(), 'line one\nand line two')
+    end)
+  end)
 
-      local range = Range.from_cmd_args(args)
+  it('from_cmd_args: range=2: visual: charwise', function()
+    local args = { range = 2, line1 = 1, line2 = 1 }
+    withbuf({ 'line one', 'and line two' }, function()
+      -- Simulate a visual selection:
+      local a = Pos.new(nil, 1, 1)
+      local b = Pos.new(nil, 1, 4)
+      a:save_to_pos "'<"
+      b:save_to_pos "'>"
+      Range.new(a, b, 'v'):set_visual_selection()
+      assert.are.same(vim.fn.mode(), 'v')
+
+      -- In this simulated setup, we need to force visualmode() to return
+      -- 'v' and histget() to return [['<,'>]]:
+
+      -- visualmode()
+      local orig_visualmode = vim.fn.visualmode
+      --- @diagnostic disable-next-line: duplicate-set-field
+      function vim.fn.visualmode() return 'v' end
+      assert.are.same(vim.fn.visualmode(), 'v')
+
+      -- histget()
+      local orig_histget = vim.fn.histget
+      --- @diagnostic disable-next-line: duplicate-set-field
+      function vim.fn.histget(x, y) return [['<,'>]] end
+
+      -- Now run the test:
+      local range = Range.from_cmd_args(args) --[[@as u.Range]]
       assert.are.same(range.start, a)
       assert.are.same(range.stop, b)
       assert.are.same(range.mode, 'v')
+      assert.are.same(range:text(), 'line')
+
+      -- Reset visualmode() and histget():
+      vim.fn.visualmode = orig_visualmode
+      vim.fn.histget = orig_histget
     end)
   end)
 
@@ -617,4 +810,86 @@ describe('Range', function()
       }, vim.api.nvim_buf_get_lines(b, 0, -1, false))
     end)
   end)
+
+  it('can save to extmark', function()
+    withbuf({
+      'The quick brown',
+      'fox',
+      'jumps',
+      'over',
+      'the lazy dog',
+    }, function()
+      -- Construct a range over 'fox jumps'
+      local r = Range.new(Pos.new(nil, 2, 1), Pos.new(nil, 3, 5), 'v')
+      local extrange = r:save_to_extmark()
+      assert.are.same({ 'fox', 'jumps' }, extrange:range():lines())
+      -- change 'jumps' to 'leaps':
+      vim.api.nvim_buf_set_text(extrange.bufnr, 2, 0, 2, 4, { 'leap' })
+      assert.are.same({
+        'The quick brown',
+        'fox',
+        'leaps',
+        'over',
+        'the lazy dog',
+      }, vim.api.nvim_buf_get_lines(extrange.bufnr, 0, -1, false))
+      assert.are.same({ 'fox', 'leaps' }, extrange:range():lines())
+    end)
+  end)
+
+  it('can save linewise extmark', function()
+    withbuf({
+      'The quick brown',
+      'fox',
+      'jumps',
+      'over',
+      'the lazy dog',
+    }, function()
+      -- Construct a range over 'fox jumps'
+      local r = Range.new(Pos.new(nil, 2, 1), Pos.new(nil, 3, Pos.MAX_COL), 'V')
+      local extrange = r:save_to_extmark()
+      assert.are.same({ 'fox', 'jumps' }, extrange:range():lines())
+
+      local extmark = vim.api.nvim_buf_get_extmark_by_id(
+        extrange.bufnr,
+        vim.api.nvim_create_namespace 'u.range',
+        extrange.id,
+        { details = true }
+      )
+      local row0, col0, details = unpack(extmark)
+      assert.are.same({ 1, 0 }, { row0, col0 })
+      assert.are.same({ 3, 0 }, { details.end_row, details.end_col })
+    end)
+  end)
+
+  it('discerns range bounds from extmarks beyond the end of the buffer', function()
+    local Buffer = require 'u.buffer'
+
+    vim.cmd.vnew()
+    local left = Buffer.current()
+    left:set_tmp_options()
+    vim.cmd.vnew()
+    local right = Buffer.current()
+    right:set_tmp_options()
+
+    left:all():replace {
+      'one',
+      'two',
+      'three',
+    }
+    local left_all_ext = left:all():save_to_extmark()
+
+    right:all():replace {
+      'foo',
+      'bar',
+    }
+
+    vim.api.nvim_set_current_buf(right.bufnr)
+    vim.cmd [[normal! ggyG]]
+    vim.api.nvim_set_current_buf(left.bufnr)
+    vim.cmd [[normal! ggVGp]]
+
+    assert.are.same({ 'foo', 'bar' }, left_all_ext:range():lines())
+    vim.api.nvim_buf_delete(left.bufnr, { force = true })
+    vim.api.nvim_buf_delete(right.bufnr, { force = true })
+  end)
 end)
diff --git a/spec/renderer_spec.lua b/spec/renderer_spec.lua
index 8e38a2d..a4d5833 100644
--- a/spec/renderer_spec.lua
+++ b/spec/renderer_spec.lua
@@ -83,13 +83,13 @@ describe('Renderer', function()
   end)
 
   --
-  -- get_pos_infos
+  -- get_tags_at
   --
 
   it('should return no extmarks for an empty buffer', function()
     withbuf({}, function()
       local r = R.Renderer.new(0)
-      local pos_infos = r:get_pos_infos { 0, 0 }
+      local pos_infos = r:get_tags_at { 0, 0 }
       assert.are.same(pos_infos, {})
     end)
   end)
@@ -102,12 +102,28 @@ describe('Renderer', function()
         R.h('text', { hl = 'HighlightGroup2' }, ' World'),
       }
 
-      local pos_infos = r:get_pos_infos { 0, 2 }
-
+      local pos_infos = r:get_tags_at { 0, 2 }
       assert.are.same(#pos_infos, 1)
       assert.are.same(pos_infos[1].tag.attributes.hl, 'HighlightGroup1')
       assert.are.same(pos_infos[1].extmark.start, { 0, 0 })
       assert.are.same(pos_infos[1].extmark.stop, { 0, 5 })
+      pos_infos = r:get_tags_at { 0, 4 }
+      assert.are.same(#pos_infos, 1)
+      assert.are.same(pos_infos[1].tag.attributes.hl, 'HighlightGroup1')
+      assert.are.same(pos_infos[1].extmark.start, { 0, 0 })
+      assert.are.same(pos_infos[1].extmark.stop, { 0, 5 })
+
+      pos_infos = r:get_tags_at { 0, 5 }
+      assert.are.same(#pos_infos, 1)
+      assert.are.same(pos_infos[1].tag.attributes.hl, 'HighlightGroup2')
+      assert.are.same(pos_infos[1].extmark.start, { 0, 5 })
+      assert.are.same(pos_infos[1].extmark.stop, { 0, 11 })
+
+      -- In insert mode, bounds are eagerly included:
+      pos_infos = r:get_tags_at({ 0, 5 }, 'i')
+      assert.are.same(#pos_infos, 2)
+      assert.are.same(pos_infos[1].tag.attributes.hl, 'HighlightGroup1')
+      assert.are.same(pos_infos[2].tag.attributes.hl, 'HighlightGroup2')
     end)
   end)
 
@@ -125,11 +141,122 @@ describe('Renderer', function()
         }),
       }
 
-      local pos_infos = r:get_pos_infos { 0, 5 }
+      local pos_infos = r:get_tags_at { 0, 5 }
 
       assert.are.same(#pos_infos, 2)
       assert.are.same(pos_infos[1].tag.attributes.hl, 'HighlightGroup2')
       assert.are.same(pos_infos[2].tag.attributes.hl, 'HighlightGroup1')
     end)
   end)
+
+  it('repeated patch_lines calls should not change the buffer content', function()
+    local lines = {
+      [[{ {]],
+      [[    bounds = {]],
+      [[      start1 = { 1, 1 },]],
+      [[      stop1 = { 4, 1 }]],
+      [[    },]],
+      [[    end_right_gravity = true,]],
+      [[    id = 1,]],
+      [[    ns_id = 623,]],
+      [[    ns_name = "my.renderer:91",]],
+      [[    right_gravity = false]],
+      [[  } }]],
+      [[]],
+    }
+    withbuf(lines, function()
+      local Buffer = require 'u.buffer'
+      R.Renderer.patch_lines(0, nil, lines)
+      assert.are.same(Buffer.current():all():lines(), lines)
+
+      R.Renderer.patch_lines(0, lines, lines)
+      assert.are.same(Buffer.current():all():lines(), lines)
+
+      R.Renderer.patch_lines(0, lines, lines)
+      assert.are.same(Buffer.current():all():lines(), lines)
+    end)
+  end)
+
+  it('should fire text-changed events', function()
+    withbuf({}, function()
+      local Buffer = require 'u.buffer'
+      local buf = Buffer.current()
+      local r = R.Renderer.new(0)
+      local captured_changed_text = ''
+      r:render {
+        R.h('text', {
+          on_change = function(txt) captured_changed_text = txt end,
+        }, {
+          'one\n',
+          'two\n',
+          'three\n',
+        }),
+      }
+
+      vim.fn.setreg('"', 'bleh')
+      vim.cmd [[normal! ggVGp]]
+      -- For some reason, the autocmd does not fire in the busted environment.
+      -- We'll call the handler ourselves:
+      r:_on_text_changed()
+
+      assert.are.same(buf:all():text(), 'bleh')
+      assert.are.same(captured_changed_text, 'bleh')
+    end)
+  end)
+
+  it('should find tags by position', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render {
+        'pre',
+        R.h('text', {
+          id = 'outer',
+        }, {
+          'inner-pre',
+          R.h('text', {
+            id = 'inner',
+          }, {
+            'inner-text',
+          }),
+          'inner-post',
+        }),
+        'post',
+      }
+
+      local tags = r:get_tags_at { 0, 11 }
+      assert.are.same(#tags, 1)
+      assert.are.same(tags[1].tag.attributes.id, 'outer')
+
+      tags = r:get_tags_at { 0, 12 }
+      assert.are.same(#tags, 2)
+      assert.are.same(tags[1].tag.attributes.id, 'inner')
+      assert.are.same(tags[2].tag.attributes.id, 'outer')
+    end)
+  end)
+
+  it('should find tags by id', function()
+    withbuf({}, function()
+      local r = R.Renderer.new(0)
+      r:render {
+        R.h('text', {
+          id = 'outer',
+        }, {
+          'inner-pre',
+          R.h('text', {
+            id = 'inner',
+          }, {
+            'inner-text',
+          }),
+          'inner-post',
+        }),
+        'post',
+      }
+
+      local bounds = r:get_tag_bounds 'outer'
+      assert.are.same(bounds, { start = { 0, 0 }, stop = { 0, 29 } })
+
+      bounds = r:get_tag_bounds 'inner'
+      assert.are.same(bounds, { start = { 0, 9 }, stop = { 0, 19 } })
+    end)
+  end)
 end)