perf(treesitter): filter out trees outside the visible range early

Problem:
Treesitter highlighter's on_line was iterating all the parsed trees,
which can be quite a lot when injection is used. This may slow down
scrolling and cursor movement in big files with many comment injections
(e.g., lsp/_meta/protocol.lua).

Solution:
In on_win, collect trees inside the visible range, and use them in
on_line.

NOTE:
This optimization depends on the correctness of on_win's botline_guess
parameter (i.e., it's always greater than or equal to the line numbers
passed to on_line). The documentation does not guarantee this, but I
have never noticed a problem so far.

runtime/lua/vim/treesitter/highlighter.lua

@@ -5,14 +5,16 @@ local Range = require('vim.treesitter._range')
 ---@alias TSHlIter fun(end_line: integer|nil): integer, TSNode, TSMetadata
 
 ---@class TSHighlightState
+---@field tstree TSTree
 ---@field next_row integer
 ---@field iter TSHlIter|nil
+---@field highlighter_query TSHighlighterQuery
 
 ---@class TSHighlighter
 ---@field active table<integer,TSHighlighter>
 ---@field bufnr integer
 ---@field orig_spelloptions string
----@field _highlight_states table<TSTree,TSHighlightState>
+---@field _highlight_states TSHighlightState[]
 ---@field _queries table<string,TSHighlighterQuery>
 ---@field tree LanguageTree
 ---@field redraw_count integer
@@ -157,18 +159,47 @@ function TSHighlighter:destroy()
   end
 end
 
----@package
----@param tstree TSTree
----@return TSHighlightState
-function TSHighlighter:get_highlight_state(tstree)
-  if not self._highlight_states[tstree] then
-    self._highlight_states[tstree] = {
+---@param srow integer
+---@param erow integer exclusive
+---@private
+function TSHighlighter:prepare_highlight_states(srow, erow)
+  self.tree:for_each_tree(function(tstree, tree)
+    if not tstree then
+      return
+    end
+
+    local root_node = tstree:root()
+    local root_start_row, _, root_end_row, _ = root_node:range()
+
+    -- Only worry about trees within the visible range
+    if root_start_row > erow or root_end_row < srow then
+      return
+    end
+
+    local highlighter_query = self:get_query(tree:lang())
+
+    -- Some injected languages may not have highlight queries.
+    if not highlighter_query:query() then
+      return
+    end
+
+    -- _highlight_states should be a list so that the highlights are added in the same order as
+    -- for_each_tree traversal. This ensures that parents' highlight don't override children's.
+    table.insert(self._highlight_states, {
+      tstree = tstree,
       next_row = 0,
       iter = nil,
-    }
-  end
+      highlighter_query = highlighter_query,
+    })
+  end)
+end
 
-  return self._highlight_states[tstree]
+---@param fn fun(state: TSHighlightState)
+---@package
+function TSHighlighter:for_each_highlight_state(fn)
+  for _, state in ipairs(self._highlight_states) do
+    fn(state)
+  end
 end
 
 ---@private
@@ -214,12 +245,8 @@ end
 ---@param line integer
 ---@param is_spell_nav boolean
 local function on_line_impl(self, buf, line, is_spell_nav)
-  self.tree:for_each_tree(function(tstree, tree)
-    if not tstree then
-      return
-    end
-
-    local root_node = tstree:root()
+  self:for_each_highlight_state(function(state)
+    local root_node = state.tstree:root()
     local root_start_row, _, root_end_row, _ = root_node:range()
 
     -- Only worry about trees within the line range
@@ -227,17 +254,9 @@ local function on_line_impl(self, buf, line, is_spell_nav)
       return
     end
 
-    local state = self:get_highlight_state(tstree)
-    local highlighter_query = self:get_query(tree:lang())
-
-    -- Some injected languages may not have highlight queries.
-    if not highlighter_query:query() then
-      return
-    end
-
     if state.iter == nil or state.next_row < line then
       state.iter =
-        highlighter_query:query():iter_captures(root_node, self.bufnr, line, root_end_row + 1)
+        state.highlighter_query:query():iter_captures(root_node, self.bufnr, line, root_end_row + 1)
     end
 
     while line >= state.next_row do
@@ -250,9 +269,9 @@ local function on_line_impl(self, buf, line, is_spell_nav)
       local start_row, start_col, end_row, end_col = Range.unpack4(range)
 
       if capture then
-        local hl = highlighter_query.hl_cache[capture]
+        local hl = state.highlighter_query.hl_cache[capture]
 
-        local capture_name = highlighter_query:query().captures[capture]
+        local capture_name = state.highlighter_query:query().captures[capture]
         local spell = nil ---@type boolean?
         if capture_name == 'spell' then
           spell = true
@@ -327,6 +346,7 @@ function TSHighlighter._on_win(_, _win, buf, topline, botline)
   end
   self.tree:parse({ topline, botline + 1 })
   self:reset_highlight_state()
+  self:prepare_highlight_states(topline, botline + 1)
   self.redraw_count = self.redraw_count + 1
   return true
 end