diff --git a/lua/u/renderer.lua b/lua/u/renderer.lua
index 73abb8c..3a9a13d 100644
--- a/lua/u/renderer.lua
+++ b/lua/u/renderer.lua
@@ -1,13 +1,25 @@
+---@diagnostic disable: redefined-local
 function _G.URendererOpFuncSwallow() end
 
-local ENABLE_LOG = false
+--- Comma-separated list of regex patterns for debug logging. Examples:
+--- "render,extmark" or ".*" or "get_tags_at,_on_text_changed" or
+--- "^_on_.*,reconcile$"
+local DEBUG = vim.env.DEBUG or ''
 
-local function log(...)
-  if not ENABLE_LOG then return end
+--- @param name string
+local function log(name, ...)
+  if DEBUG == '' then return end
 
-  local f = assert(io.open(vim.fs.joinpath(vim.fn.stdpath 'log', 'u.renderer.log'), 'a+'))
-  f:write(os.date() .. '\t' .. vim.inspect { ... } .. '\n')
-  f:close()
+  for pattern in DEBUG:gmatch '[^,]+' do
+    if name:match(vim.trim(pattern)) then
+      local f = assert(
+        io.open(vim.fs.joinpath((vim.fn.stdpath 'log') --[[@as string]], 'u.renderer.log'), 'a+')
+      )
+      f:write(os.date() .. '\t' .. vim.inspect { name, ... } .. '\n')
+      f:close()
+      return
+    end
+  end
 end
 
 local M = {}
@@ -15,19 +27,52 @@ local H = {}
 
 --- @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 }
+--- @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), key?: string|number }
+
+--- @generic TProps
+--- @generic TState
+--- @class u.renderer.FnComponentContext<TProps, TState>
+--- @field phase 'mount'|'update'|'unmount'
+--- @field props TProps
+--- @field state? TState
+--- @field children u.renderer.Tree
+--- @field private on_change? fun(): any
+--- @field private prev_rendered_children? u.renderer.Tree
+local FnComponentContext = {}
+M.FnComponentContext = FnComponentContext
+FnComponentContext.__index = FnComponentContext
+
+--- @param props TProps
+--- @param state? TState
+--- @param children u.renderer.Tree
+function FnComponentContext.new(props, state, children)
+  return setmetatable({
+    phase = 'mount',
+    props = props,
+    state = state,
+    children = children,
+  }, FnComponentContext)
+end
+
+--- @param new_state TState
+function FnComponentContext:update(new_state)
+  self.state = new_state
+  if self.on_change then vim.schedule(function() self.on_change() end) end
+end
+
+--- @alias u.renderer.FnComponent<TProps, TState> fun(ctx: u.renderer.FnComponentContext<TProps, TState>): u.renderer.Tree
 
 --- @class u.renderer.Tag
 --- @field kind 'tag'
---- @field name string
+--- @field name string | u.renderer.FnComponent<any, any>
 --- @field attributes u.renderer.TagAttributes
 --- @field children u.renderer.Tree
+--- @field private ctx? u.renderer.FnComponentContext
 
 --- @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: 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 {
@@ -42,39 +87,77 @@ M.h = setmetatable({}, {
       return M.h('text', vim.tbl_deep_extend('force', { hl = name }, attributes), children)
     end
   end,
-})
+}) --[[@as 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>]]
 
 -- Renderer {{{
 --- @class u.renderer.RendererExtmark
---- @field id? number
---- @field start [number, number]
---- @field stop [number, number]
+--- @field id? integer
+--- @field start [integer, integer]
+--- @field stop [integer, integer]
 --- @field opts vim.api.keyset.set_extmark
 --- @field tag u.renderer.Tag
 
 --- @class u.renderer.Renderer
---- @field bufnr number
---- @field ns number
---- @field changedtick number
---- @field old { lines: string[]; extmarks: u.renderer.RendererExtmark[] }
---- @field curr { lines: string[]; extmarks: u.renderer.RendererExtmark[] }
+--- @field bufnr integer
+--- @field ns integer
+--- @field changedtick integer
+--- @field text_content { old: { lines: string[], extmarks: u.renderer.RendererExtmark[] }, curr: { lines: string[], extmarks: u.renderer.RendererExtmark[] } }
+--- @field component_tree { old: u.renderer.Tree  }
 local Renderer = {}
 Renderer.__index = Renderer
 M.Renderer = Renderer
 
 --- @private
---- @param x any
---- @return boolean
-function Renderer.is_tag(x) return type(x) == 'table' and x.kind == 'tag' end
+--- @param tree u.renderer.Tree
+--- @param visitors {
+---   nil_?: (fun(): any),
+---   boolean?: (fun(b: boolean): any),
+---   string?: (fun(s: string): any),
+---   array?: (fun(tags: u.renderer.Node[]): any),
+---   tag?: (fun(tag: u.renderer.Tag): any),
+---   component?: (fun(component: u.renderer.FnComponent, tag: u.renderer.Tag): any),
+---   unknown?: fun(tag: any): any
+--- }
+function Renderer.tree_match(tree, visitors) -- {{{
+  local function is_tag(x) return type(x) == 'table' and x.kind == 'tag' end
+  local function is_tag_arr(x) return type(x) == 'table' and not is_tag(x) end
+
+  if tree == nil then
+    return visitors.nil_ and visitors.nil_() or nil
+  elseif type(tree) == 'boolean' then
+    return visitors.boolean and visitors.boolean(tree) or nil
+  elseif type(tree) == 'string' then
+    return visitors.string and visitors.string(tree) or nil
+  elseif is_tag_arr(tree) then
+    return visitors.array and visitors.array(tree --[[@as any]]) or nil
+  elseif is_tag(tree) then
+    local tag = tree --[[@as u.renderer.Tag]]
+    if type(tag.name) == 'function' then
+      return visitors.component and visitors.component(tag.name --[[@as function]], tag) or nil
+    else
+      return visitors.tag and visitors.tag(tree --[[@as any]]) or nil
+    end
+  else
+    return visitors.unknown and visitors.unknown(tree) or error 'unknown value: not a tag'
+  end
+end -- }}}
 
 --- @private
---- @param x any
---- @return boolean
-function Renderer.is_tag_arr(x)
-  if type(x) ~= 'table' then return false end
-  return #x == 0 or not Renderer.is_tag(x)
-end
---- @param bufnr number|nil
+--- @param tree u.renderer.Tree
+--- @return 'nil' | 'boolean' | 'string' | 'array' | 'tag' | u.renderer.FnComponent | 'unknown'
+function Renderer.tree_kind(tree) -- {{{
+  return Renderer.tree_match(tree, {
+    nil_ = function() return 'nil' end,
+    boolean = function() return 'boolean' end,
+    string = function() return 'string' end,
+    array = function() return 'array' end,
+    tag = function() return 'tag' end,
+    component = function(c) return c end,
+    unknown = function() return 'unknown' end,
+  }) --[[@as any]]
+end -- }}}
+
+--- @param bufnr integer|nil
 function Renderer.new(bufnr) -- {{{
   if bufnr == nil or bufnr == 0 then bufnr = vim.api.nvim_get_current_buf() end
 
@@ -86,8 +169,15 @@ function Renderer.new(bufnr) -- {{{
     bufnr = bufnr,
     ns = vim.b[bufnr]._renderer_ns,
     changedtick = 0,
-    old = { lines = {}, extmarks = {} },
-    curr = { lines = {}, extmarks = {} },
+    text_content = {
+      old = { lines = {}, extmarks = {} },
+      curr = { lines = {}, extmarks = {} },
+    },
+    component_tree = {
+      old = nil,
+      curr = nil,
+      ctx_by_node = {},
+    },
   }, Renderer)
 
   vim.api.nvim_create_autocmd({ 'TextChanged', 'TextChangedI', 'TextChangedP' }, {
@@ -99,8 +189,8 @@ function Renderer.new(bufnr) -- {{{
 end -- }}}
 
 --- @param opts {
----   tree: u.renderer.Tree;
----   on_tag?: fun(tag: u.renderer.Tag, start0: [number, number], stop0: [number, number]): any;
+---   tree: u.renderer.Tree,
+---   on_tag?: fun(tag: u.renderer.Tag, start0: [number, number], stop0: [number, number]): any
 --- }
 function Renderer.markup_to_lines(opts) -- {{{
   --- @type string[]
@@ -119,39 +209,31 @@ function Renderer.markup_to_lines(opts) -- {{{
     curr_col1 = 1
   end
 
-  --- @param node u.renderer.Node
+  --- @param node u.renderer.Tree
   local function visit(node) -- {{{
-    if node == nil or type(node) == 'boolean' then return end
-
-    if type(node) == 'string' then
-      local node_lines = vim.split(node, '\n')
-      for lnum, s in ipairs(node_lines) do
-        if lnum > 1 then put_line() end
-        put(s)
-      end
-    elseif Renderer.is_tag(node) then
-      local start0 = { curr_line1 - 1, curr_col1 - 1 }
-
-      -- visit the children:
-      if Renderer.is_tag_arr(node.children) then
-        for _, child in
-          ipairs(node.children --[[@as u.renderer.Node[] ]])
-        do
-          -- newlines are not controlled by array entries, do NOT output a line here:
+    Renderer.tree_match(node, {
+      string = function(s_node)
+        local node_lines = vim.split(s_node, '\n')
+        for lnum, s in ipairs(node_lines) do
+          if lnum > 1 then put_line() end
+          put(s)
+        end
+      end,
+      array = function(ts)
+        for _, child in ipairs(ts) do
           visit(child)
         end
-      else -- luacheck: ignore
-        visit(node.children)
-      end
+      end,
+      tag = function(t)
+        assert(type(t.name) ~= 'function', 'markup_to_lines does not render components')
 
-      local stop0 = { curr_line1 - 1, curr_col1 - 1 }
-      if opts.on_tag then opts.on_tag(node, start0, stop0) end
-    elseif Renderer.is_tag_arr(node) then
-      for _, child in ipairs(node) do
-        -- newlines are not controlled by array entries, do NOT output a line here:
-        visit(child)
-      end
-    end
+        local start0 = { curr_line1 - 1, curr_col1 - 1 }
+        visit(t.children)
+        local stop0 = { curr_line1 - 1, curr_col1 - 1 }
+
+        if opts.on_tag then opts.on_tag(t, start0, stop0) end
+      end,
+    })
   end -- }}}
   visit(opts.tree)
 
@@ -159,12 +241,12 @@ function Renderer.markup_to_lines(opts) -- {{{
 end -- }}}
 
 --- @param opts {
----   tree: u.renderer.Tree;
----   format_tag?: fun(tag: u.renderer.Tag): 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
 
---- @param bufnr number
+--- @param bufnr integer
 --- @param old_lines string[] | nil
 --- @param new_lines string[]
 function Renderer.patch_lines(bufnr, old_lines, new_lines) -- {{{
@@ -190,8 +272,10 @@ function Renderer.patch_lines(bufnr, old_lines, new_lines) -- {{{
   end
 
   -- Morph the text to the desired state:
-  local line_changes =
-    H.levenshtein(old_lines or vim.api.nvim_buf_get_lines(bufnr, 0, -1, false), new_lines)
+  local line_changes = H.levenshtein {
+    from = old_lines or vim.api.nvim_buf_get_lines(bufnr, 0, -1, false),
+    to = new_lines,
+  }
   for _, line_change in ipairs(line_changes) do
     local lnum0 = line_change.index - 1
 
@@ -200,7 +284,7 @@ function Renderer.patch_lines(bufnr, old_lines, new_lines) -- {{{
     elseif line_change.kind == 'change' then
       -- Compute inter-line diff, and apply:
       local col_changes =
-        H.levenshtein(vim.split(line_change.from, ''), vim.split(line_change.to, ''))
+        H.levenshtein { from = vim.split(line_change.from, ''), to = vim.split(line_change.to, '') }
 
       for _, col_change in ipairs(col_changes) do
         local cnum0 = col_change.index - 1
@@ -226,7 +310,8 @@ end -- }}}
 function Renderer:render(tree) -- {{{
   local changedtick = vim.b[self.bufnr].changedtick
   if changedtick ~= self.changedtick then
-    self.curr = { lines = vim.api.nvim_buf_get_lines(self.bufnr, 0, -1, false) }
+    self.text_content.curr =
+      { extmarks = {}, lines = vim.api.nvim_buf_get_lines(self.bufnr, 0, -1, false) }
     self.changedtick = changedtick
   end
 
@@ -283,8 +368,8 @@ function Renderer:render(tree) -- {{{
     end, -- }}}
   }
 
-  self.old = self.curr
-  self.curr = { lines = lines, extmarks = extmarks }
+  self.text_content.old = self.text_content.curr
+  self.text_content.curr = { lines = lines, extmarks = extmarks }
   self:_reconcile()
   vim.cmd.doautocmd { args = { 'User', 'Renderer:' .. tostring(self.bufnr) .. ':render' } }
 end -- }}}
@@ -294,7 +379,7 @@ function Renderer:_reconcile() -- {{{
   --
   -- Step 1: morph the text to the desired state:
   --
-  Renderer.patch_lines(self.bufnr, self.old.lines, self.curr.lines)
+  Renderer.patch_lines(self.bufnr, self.text_content.old.lines, self.text_content.curr.lines)
   self.changedtick = vim.b[self.bufnr].changedtick
 
   --
@@ -309,7 +394,7 @@ function Renderer:_reconcile() -- {{{
   vim.api.nvim_buf_clear_namespace(self.bufnr, self.ns, 0, -1)
 
   -- Set current extmarks:
-  for _, extmark in ipairs(self.curr.extmarks) do
+  for _, extmark in ipairs(self.text_content.curr.extmarks) do
     extmark.id = vim.api.nvim_buf_set_extmark(
       self.bufnr,
       self.ns,
@@ -330,7 +415,7 @@ function Renderer:_reconcile() -- {{{
     )
   end
 
-  self.old = self.curr
+  self.text_content.old = self.text_content.curr
 end -- }}}
 
 --- @private
@@ -340,9 +425,9 @@ 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
-  log('_expr_map_callback: pos0:', pos0)
+  log('_expr_map_callback', 'pos0:', pos0)
   local pos_infos = self:get_tags_at(pos0)
-  log('_expr_map_callback: pos_infos:', pos_infos)
+  log('_expr_map_callback', 'pos_infos:', pos_infos)
 
   if #pos_infos == 0 then return lhs end
 
@@ -392,7 +477,7 @@ function Renderer:_on_text_changed() -- {{{
       --- @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
-      log('_on_text_changed: fetched current extmark for pos_info', {
+      log('_on_text_changed', 'fetched current extmark for pos_info', {
         pos_info = pos_info,
         curr_extmark = {
           start_row0 = start_row0,
@@ -421,7 +506,7 @@ function Renderer:_on_text_changed() -- {{{
           or ''
         end_col0 = last_line:len()
       end
-      log('_on_text_changed: after position correction', {
+      log('_on_text_changed', 'after position correction', {
         curr_extmark = {
           start_row0 = start_row0,
           start_col0 = start_col0,
@@ -439,7 +524,7 @@ function Renderer:_on_text_changed() -- {{{
       local pos2 = { self.bufnr, end_row0 + 1, end_col0 }
       local ok, lines = pcall(vim.fn.getregion, pos1, pos2, { type = 'v' })
       if not ok then
-        log('_on_text_changed: getregion: invalid-pos ', {
+        log('_on_text_changed', 'getregion: invalid-pos ', {
           { pos1, pos2 },
         })
         vim.api.nvim_echo({
@@ -492,7 +577,7 @@ function Renderer:_debug() -- {{{
         ),
       },
       computed = {
-        extmarks = self.curr.extmarks,
+        extmarks = self.text_content.curr.extmarks,
       },
     }
     vim.api.nvim_buf_set_lines(info_bufnr, 0, -1, true, vim.split(vim.inspect(info), '\n'))
@@ -534,9 +619,9 @@ end -- }}}
 --- (outermost).
 ---
 --- @private (private for now)
---- @param pos0 [number; number]
+--- @param pos0 [integer, integer]
 --- @param mode string?
---- @return { extmark: u.renderer.RendererExtmark; tag: u.renderer.Tag; }[]
+--- @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
@@ -550,7 +635,8 @@ function Renderer:get_tags_at(pos0, mode) -- {{{
     { details = true, overlap = true }
   )
   log(
-    'get_tags_at: context:',
+    'get_tags_at',
+    'context:',
     { pos0 = pos0, mode = mode, raw_overlapping_extmarks = raw_overlapping_extmarks }
   )
 
@@ -560,58 +646,65 @@ function Renderer:get_tags_at(pos0, mode) -- {{{
   --- @type u.renderer.RendererExtmark[]
   local mapped_extmarks = vim
     .iter(raw_overlapping_extmarks)
-    --- @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)
-      local start = { line0, col0 }
-      local stop = { line0, col0 }
-      if details and details.end_row ~= nil and details.end_col ~= nil then
-        stop = { details.end_row, details.end_col }
+    :map(
+      --- @return u.renderer.RendererExtmark
+      function(ext)
+        --- @type integer, integer, integer, { end_row?: number, end_col?: number }|nil
+        local id, line0, col0, details = unpack(ext)
+        local start = { line0, col0 }
+        local stop = { line0, col0 }
+        if details and details.end_row ~= nil and details.end_col ~= nil then
+          stop = {
+            details.end_row --[[@as integer]],
+            details.end_col --[[@as integer]],
+          }
+        end
+        return { id = id, start = start, stop = stop, opts = details }
       end
-      return { id = id, start = start, stop = stop, opts = details }
-    end)
+    )
     :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
-        -- 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
+    :filter(
+      --- @param ext u.renderer.RendererExtmark
+      function(ext)
+        if ext.stop[1] ~= nil and ext.stop[2] ~= nil then
+          -- 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
-
-        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
-    end)
+    )
     :totable()
 
   -- Sort the tags into smallest (inner) to largest (outer):
@@ -637,18 +730,20 @@ function Renderer:get_tags_at(pos0, mode) -- {{{
   )
 
   -- When we set the extmarks in the step above, we captured the IDs of the
-  -- created extmarks in self.curr.extmarks, which also has which tag each
+  -- created extmarks in self.text_content.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: u.renderer.RendererExtmark; tag: u.renderer.Tag }[]
+  --- @type { extmark: u.renderer.RendererExtmark, tag: u.renderer.Tag }[]
   local matching_tags = vim
     .iter(intersecting_extmarks)
-    --- @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
+    :map(
+      --- @param ext u.renderer.RendererExtmark
+      function(ext)
+        for _, extmark_cache in ipairs(self.text_content.curr.extmarks) do
+          if extmark_cache.id == ext.id then return { extmark = ext, tag = extmark_cache.tag } end
+        end
       end
-    end)
+    )
     :totable()
 
   return matching_tags
@@ -656,9 +751,9 @@ end -- }}}
 
 --- @private
 --- @param tag_or_id string | u.renderer.Tag
---- @return { start: [number, number]; stop: [number, number] } | nil
+--- @return { start: [number, number], stop: [number, number] } | nil
 function Renderer:get_tag_bounds(tag_or_id) -- {{{
-  for _, x in ipairs(self.curr.extmarks) do
+  for _, x in ipairs(self.text_content.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
@@ -666,6 +761,189 @@ function Renderer:get_tag_bounds(tag_or_id) -- {{{
   end
 end -- }}}
 
+--- @param tree u.renderer.Tree
+function Renderer:mount(tree) -- {{{
+  local function rerender()
+    local H2 = {}
+
+    --- @param tree u.renderer.Tree
+    function H2.unmount(tree)
+      log('Render.mount.umount', { tree = tree })
+      --- @param tree u.renderer.Tree
+      local function visit(tree)
+        Renderer.tree_match(tree, {
+          array = function(tags)
+            log('Render.mount.umount.array', { tags = tags })
+            for _, tag in ipairs(tags) do
+              visit(tag)
+            end
+          end,
+          tag = function(tag)
+            log('Render.mount.umount.tag', { tag = tag })
+            visit(tag.children)
+          end,
+          component = function(Component, tag)
+            log('Render.mount.umount.component', { Component = Component, tag = tag })
+            --- @type u.renderer.FnComponentContext
+            local ctx = assert(tag.ctx, 'could not find context for node')
+
+            -- depth-first:
+            visit(ctx.children)
+
+            -- now unmount current:
+            ctx.phase = 'unmount'
+            Component(ctx)
+            ctx.on_change = nil
+          end,
+        })
+      end
+
+      visit(tree)
+    end
+
+    --- @param old_tree u.renderer.Tree
+    --- @param new_tree u.renderer.Tree
+    --- @return u.renderer.Tree
+    function H2.visit_tree(old_tree, new_tree)
+      local old_tree_kind = Renderer.tree_kind(old_tree)
+      local new_tree_kind = Renderer.tree_kind(new_tree)
+      log('Renderer.mount.visit_tree', {
+        old_tree_kind = old_tree_kind,
+        old_tree = old_tree,
+        new_tree_kind = new_tree_kind,
+        new_tree = new_tree,
+      })
+
+      local new_tree_rendered = Renderer.tree_match(new_tree, {
+        string = function(s) return s end,
+        boolean = function(b) return b end,
+        nil_ = function() return nil end,
+
+        array = function(new_arr)
+          return H2.visit_array(old_tree --[[@as any]], new_arr)
+        end,
+        tag = function(new_tag)
+          log('Renderer.mount.visit_tree.tag', { new_tag = new_tag })
+          local old_children = old_tree_kind == new_tree_kind and old_tree.children or nil
+          return M.h(
+            new_tag.name,
+            new_tag.attributes,
+            H2.visit_tree(old_children --[[@as any]], new_tag.children --[[@as any]])
+          )
+        end,
+
+        component = function(NewC, new_tag)
+          --- @type { tag: u.renderer.Tag, ctx?: u.renderer.FnComponentContext } | nil
+          local old_component_info = Renderer.tree_match(
+            old_tree,
+            { component = function(_, t) return { tag = t, ctx = t.ctx } end }
+          )
+          local old_tag = old_component_info and old_component_info.tag or nil
+          local ctx = old_component_info and old_component_info.ctx or nil
+
+          log('Renderer.mount.visit_tree.component', {
+            NewC = NewC,
+            new_tag = new_tag,
+            old_tag = old_tag,
+          })
+
+          if not ctx then
+            --- @type u.renderer.FnComponentContext
+            ctx = FnComponentContext.new(new_tag.attributes, nil, new_tag.children)
+          else
+            ctx.phase = 'update'
+          end
+          ctx.props = new_tag.attributes
+          ctx.children = new_tag.children
+          ctx.on_change = rerender
+
+          new_tag.ctx = ctx
+          local NewC_rendered_children = NewC(ctx)
+          local result = H2.visit_tree(ctx.prev_rendered_children, NewC_rendered_children)
+          ctx.prev_rendered_children = NewC_rendered_children
+          return result
+        end,
+      })
+
+      if old_tree_kind ~= new_tree_kind then H2.unmount(old_tree) end
+
+      return new_tree_rendered
+    end
+
+    --- @param old_arr u.renderer.Node[]
+    --- @param new_arr u.renderer.Node[]
+    --- @return u.renderer.Node[]
+    function H2.visit_array(old_arr, new_arr)
+      -- We are going to hijack levenshtein in order to compute the
+      -- difference between elements/components. In this model, we need to
+      -- "update" all the nodes, so no nodes are equal. We will rely on
+      -- levenshtein to find the "shortest path" to conforming old => new via
+      -- the cost.of_change function. That will provide the meat of modeling
+      -- what effort it will take to morph one element into the new form.
+      -- What levenshtein gives us for free in this model is also informing
+      -- us what needs to be added (i.e., "mounted"), what needs to be
+      -- deleted ("unmounted") and what needs to be changed ("updated").
+      local changes = H.levenshtein {
+        from = old_arr or {},
+        to = new_arr or {},
+        are_equal = function() return false end,
+        cost = {
+          of_change = function(tree1, tree2, tree1_idx, tree2_idx)
+            --- @return string
+            local function verbose_tree_kind(tree, idx)
+              return Renderer.tree_match(tree, {
+                nil_ = function() return 'nil' end,
+                string = function() return 'string' end,
+                boolean = function() return 'boolean' end,
+                array = function() return 'array' end,
+                tag = function(tag)
+                  return ('tag-%s-%s'):format(tag.name, tostring(tag.attributes.key or idx))
+                end,
+                component = function(C, tag)
+                  return ('component-%s-%s'):format(C, tostring(tag.attributes.key or idx))
+                end,
+              })
+            end
+            local tree1_inf = verbose_tree_kind(tree1, tree1_idx)
+            local tree2_inf = verbose_tree_kind(tree2, tree2_idx)
+            return tree1_inf == tree2_inf and 1 or 2
+          end,
+        },
+      }
+
+      --- @type u.renderer.Node[]
+      local resulting_nodes = {}
+      for _, change in ipairs(changes) do
+        local resulting_node
+        if change.kind == 'add' then
+          resulting_node = H2.visit_tree(nil, change.item)
+        elseif change.kind == 'delete' then
+          H2.visit_tree(change.item, nil)
+        elseif change.kind == 'change' then
+          resulting_node = H2.visit_tree(change.from, change.to)
+        end
+        if resulting_node then table.insert(resulting_nodes, 1, resulting_node) end
+      end
+
+      log('Renderer.mount.visit_array', {
+        old_arr = old_arr,
+        new_arr = new_arr,
+        changes = changes,
+        resulting_nodes = resulting_nodes,
+      })
+
+      return resulting_nodes
+    end
+
+    local renderable_tree = H2.visit_tree(self.component_tree.old, tree)
+    self.component_tree.old = tree
+    self:render(renderable_tree)
+  end
+
+  -- Kick off initial render:
+  rerender()
+end -- }}}
+
 -- }}}
 
 -- TreeBuilder {{{
@@ -733,14 +1011,21 @@ function TreeBuilder:tree() return self.nodes end
 
 -- Levenshtein utility {{{
 -- luacheck: ignore
---- @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; })
+--- @alias u.renderer.LevenshteinChange<T> ({ kind: 'add', item: T, index: integer } | { kind: 'delete', item: T, index: integer } | { kind: 'change', from: T, to: T, index: integer })
 --- @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; }
+--- @param opts {
+---   from: `T`[],
+---   to: T[],
+---   are_equal?: (fun(x: T, y: T, xidx: integer, yidx: integer): boolean),
+---   cost?: {
+---     of_delete?: (fun(x: T, idx: integer): number),
+---     of_add?: (fun(x: T, idx: integer): number),
+---     of_change?: (fun(x: T, y: T, xidx: integer, yidx: integer): number)
+---   }
+--- }
 --- @return u.renderer.LevenshteinChange<T>[] The changes, from last (greatest index) to first (smallest index).
-function H.levenshtein(x, y, cost)
+function H.levenshtein(opts)
   -- 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
   -- fudging with the costs of operations produces a more optimized change-set
@@ -750,12 +1035,13 @@ function H.levenshtein(x, y, cost)
   -- in here even though it's not actively used. Hopefully having this
   -- callback-based plumbing does not cause too much of a performance hit to
   -- the renderer.
-  cost = cost or {}
-  local cost_of_delete_f = cost.of_delete or function() return 1 end
-  local cost_of_add_f = cost.of_add or function() return 1 end
-  local cost_of_change_f = cost.of_change or function() return 1 end
+  if not opts.are_equal then opts.are_equal = function(x, y) return x == y end end
+  if not opts.cost then opts.cost = {} end
+  if not opts.cost.of_add then opts.cost.of_add = function() return 1 end end
+  if not opts.cost.of_change then opts.cost.of_change = function() return 1 end end
+  if not opts.cost.of_delete then opts.cost.of_delete = function() return 1 end end
 
-  local m, n = #x, #y
+  local m, n = #opts.from, #opts.to
   -- Initialize the distance matrix
   local dp = {}
   for i = 0, m do
@@ -776,12 +1062,12 @@ function H.levenshtein(x, y, cost)
   -- Compute the Levenshtein distance dynamically
   for i = 1, m do
     for j = 1, n do
-      if x[i] == y[j] then
+      if opts.are_equal(opts.from[i], opts.to[j], i, j) then
         dp[i][j] = dp[i - 1][j - 1] -- no cost if items are the same
       else
-        local cost_delete = dp[i - 1][j] + cost_of_delete_f(x[i])
-        local cost_add = dp[i][j - 1] + cost_of_add_f(y[j])
-        local cost_change = dp[i - 1][j - 1] + cost_of_change_f(x[i], y[j])
+        local cost_delete = dp[i - 1][j] + opts.cost.of_delete(opts.from[i], i)
+        local cost_add = dp[i][j - 1] + opts.cost.of_add(opts.to[j], j)
+        local cost_change = dp[i - 1][j - 1] + opts.cost.of_change(opts.from[i], opts.to[j], i, j)
         dp[i][j] = math.min(cost_delete, cost_add, cost_change)
       end
     end
@@ -806,9 +1092,9 @@ 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
+      if not opts.are_equal(opts.from[i], opts.to[j]) then
         --- @type u.renderer.LevenshteinChange
-        local change = { kind = 'change', from = x[i], index = i, to = y[j] }
+        local change = { kind = 'change', from = opts.from[i], index = i, to = opts.to[j] }
         table.insert(changes, change)
       end
       i = i - 1
@@ -816,13 +1102,13 @@ function H.levenshtein(x, y, cost)
     elseif is_first_min(cost_of_add, cost_of_change, cost_of_delete) then
       -- addition
       --- @type u.renderer.LevenshteinChange
-      local change = { kind = 'add', item = y[j], index = i + 1 }
+      local change = { kind = 'add', item = opts.to[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 u.renderer.LevenshteinChange
-      local change = { kind = 'delete', item = x[i], index = i }
+      local change = { kind = 'delete', item = opts.from[i], index = i }
       table.insert(changes, change)
       i = i - 1
     else