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/lua/u/buffer.lua b/lua/u/buffer.lua
index 0897a4f..410d1d3 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
 
diff --git a/lua/u/range.lua b/lua/u/range.lua
index 69406dd..5a96b51 100644
--- a/lua/u/range.lua
+++ b/lua/u/range.lua
@@ -91,14 +91,12 @@ function Range.from_marks(lpos, rpos)
 end
 
 --- @param bufnr number
---- @param id number
-function Range.from_extmark(bufnr, id)
-  local mode = 'v'
-
+--- @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(vim.api.nvim_buf_get_extmark_by_id(bufnr, NS, id, { details = true }))
+  local start_row0, start_col0, details = unpack(extmark)
 
+  local mode = 'v'
   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)
 
@@ -651,7 +649,14 @@ end
 
 function ExtmarkRange.new(bufnr, id) return setmetatable({ bufnr = bufnr, id = id }, ExtmarkRange) end
 
-function ExtmarkRange:range() return Range.from_extmark(self.bufnr, self.id) 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
 
diff --git a/lua/u/renderer.lua b/lua/u/renderer.lua
index 1873ecc..8696be8 100644
--- a/lua/u/renderer.lua
+++ b/lua/u/renderer.lua
@@ -1,12 +1,23 @@
+local Signal = require('u.tracker').Signal
+
 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> }
+
+--- @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 {
@@ -25,14 +36,19 @@ 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
@@ -64,6 +80,12 @@ 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 -- }}}
 
@@ -199,7 +221,7 @@ function Renderer:render(tree) -- {{{
     self.changedtick = changedtick
   end
 
-  --- @type RendererExtmark[]
+  --- @type u.renderer.RendererExtmark[]
   local extmarks = {}
 
   --- @type string[]
@@ -214,31 +236,28 @@ 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',
+              mode,
               lhs,
-              function() return self:_expr_map_callback('n', lhs) end,
+              function() return self:_expr_map_callback(mode, lhs) 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, -- }}}
   }
@@ -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:
@@ -333,20 +353,54 @@ function Renderer:_expr_map_callback(mode, lhs) -- {{{
   return cancel and '' or lhs
 end -- }}}
 
+function Renderer:_on_text_changed()
+  --- @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_pos_infos { l, c }
+  for _, pos_info in ipairs(pos_infos) do
+    local extmark_inf = pos_info.extmark
+    local tag = pos_info.tag
+    if tag.attributes.signal and getmetatable(tag.attributes.signal) == Signal then
+      --- @type u.Signal
+      local signal = tag.attributes.signal
+
+      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
+
+      if start_row0 == end_row0 and start_col0 == end_col0 then
+        -- Invalid extmark
+      else
+        local lines = vim.fn.getregion(
+          { self.bufnr, start_row0 + 1, start_col0 + 1 },
+          { self.bufnr, end_row0 + 1, end_col0 },
+          { type = 'v' }
+        )
+        local text = table.concat(lines, '\n')
+        if text ~= signal:get() then signal:schedule_set(text) end
+      end
+    end
+  end
+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; }[]
+--- @return { extmark: u.renderer.RendererExtmark; tag: u.renderer.Tag; }[]
 function Renderer:get_pos_infos(pos0) -- {{{
   local cursor_line0, cursor_col0 = pos0[1], pos0[2]
 
   -- 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[]
+  --- @type u.renderer.RendererExtmark[]
   local intersecting_extmarks = vim
     .iter(
       vim.api.nvim_buf_get_extmarks(
@@ -357,7 +411,7 @@ 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)
@@ -368,7 +422,7 @@ function Renderer:get_pos_infos(pos0) -- {{{
       end
       return { id = id, start = start, stop = stop, opts = details }
     end)
-    --- @param ext RendererExtmark
+    --- @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]
@@ -384,8 +438,8 @@ 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 +461,10 @@ 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
@@ -423,7 +477,7 @@ end -- }}}
 -- }}}
 
 -- TreeBuilder {{{
---- @class u.TreeBuilder
+--- @class u.renderer.TreeBuilder
 --- @field private nodes u.renderer.Node[]
 local TreeBuilder = {}
 TreeBuilder.__index = TreeBuilder
@@ -435,7 +489,7 @@ 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
@@ -444,7 +498,7 @@ 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)
@@ -452,7 +506,7 @@ function TreeBuilder:put_h(name, attributes, children)
 end
 
 --- @param fn fun(TreeBuilder): any
---- @return u.TreeBuilder
+--- @return u.renderer.TreeBuilder
 function TreeBuilder:nest(fn)
   local nested_writer = TreeBuilder.new()
   fn(nested_writer)
diff --git a/lua/u/utils.lua b/lua/u/utils.lua
index e60933b..b301d40 100644
--- a/lua/u/utils.lua
+++ b/lua/u/utils.lua
@@ -4,17 +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 RawCmdArgs { args: string; bang: boolean; count: number; fargs: string[]; line1: number; line2: number; mods: string; name: string; range: 0|1|2; reg: string; smods: any }
--- 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
@@ -23,22 +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'
 
@@ -50,20 +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
 
---- @param current_args RawCmdArgs
-function M.create_forward_cmd_args(current_args)
-  local args = { args = current_args.fargs }
-  if current_args.range == 1 then
-    args.range = { current_args.line1 }
-  elseif current_args.range == 2 then
-    args.range = { current_args.line1, current_args.line2 }
-  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.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 ccf3e7f..e47a675 100644
--- a/shell.nix
+++ b/shell.nix
@@ -19,5 +19,6 @@ pkgs.mkShell {
     pkgs.lua51Packages.nlua
     pkgs.neovim
     pkgs.stylua
+    pkgs.watchexec
   ];
 }