fix(highlight): split old/new treesitter parsing

Problem: highlight_treesitter concatenated all hunk lines (context, -,
+) into a single string. Mixed old/new code produced invalid syntax
(e.g. two return statements), causing treesitter error recovery to drop
captures on lines after the syntax error.

Solution: split hunk lines into two versions — new (context + added)
and old (context + deleted) — each parsed independently. Use a line_map
to resolve treesitter row indices to buffer lines, with the old version
only mapping deleted lines to avoid duplicate extmarks on context.

Also fixes three related issues exposed by the improved TS coverage:

- Replace Normal extmark with DiffsClear (explicit fg from Normal.fg).
  Normal in extmarks doesn't reliably override vim :syntax foreground.

- Reorder priority stack to DiffsClear(198) < syntax(199) < line
  bg(200) < char bg(201). TS capture groups can carry colorscheme
  backgrounds that would override diff line backgrounds at higher
  priority.

- Gate DiffsClear on per-line coverage tracking. Only clear fugitive
  syntax fg on lines where TS/vim actually produced captures, preventing
  force-clearing on lines where error recovery drops captures.

lua/diffs/highlight.lua

@@ -3,6 +3,11 @@ local M = {}
 local dbg = require('diffs.log').dbg
 local diff = require('diffs.diff')
 
+local PRIORITY_CLEAR = 198
+local PRIORITY_SYNTAX = 199
+local PRIORITY_LINE_BG = 200
+local PRIORITY_CHAR_BG = 201
+
 ---@param bufnr integer
 ---@param ns integer
 ---@param hunk diffs.Hunk
@@ -38,7 +43,7 @@ local function highlight_text(bufnr, ns, hunk, col_offset, text, lang)
     local buf_sc = col_offset + sc
     local buf_ec = col_offset + ec
 
-    local priority = lang == 'diff' and (tonumber(metadata.priority) or 100) or 200
+    local priority = lang == 'diff' and (tonumber(metadata.priority) or 100) or PRIORITY_SYNTAX
 
     pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_sr, buf_sc, {
       end_row = buf_er,
@@ -58,16 +63,21 @@ end
 
 ---@param bufnr integer
 ---@param ns integer
----@param hunk diffs.Hunk
 ---@param code_lines string[]
----@param col_offset integer?
+---@param lang string
+---@param line_map table<integer, integer>
+---@param col_offset integer
+---@param covered_lines? table<integer, true>
 ---@return integer
-local function highlight_treesitter(bufnr, ns, hunk, code_lines, col_offset)
+local function highlight_treesitter(
-  local lang = hunk.lang
+  bufnr,
-  if not lang then
+  ns,
-    return 0
+  code_lines,
-  end
+  lang,
-
+  line_map,
+  col_offset,
+  covered_lines
+)
   local code = table.concat(code_lines, '\n')
   if code == '' then
     return 0
@@ -91,41 +101,31 @@ local function highlight_treesitter(bufnr, ns, hunk, code_lines, col_offset)
     return 0
   end
 
-  if hunk.header_context and hunk.header_context_col then
-    local header_line = hunk.start_line - 1
-    pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, header_line, hunk.header_context_col, {
-      end_col = hunk.header_context_col + #hunk.header_context,
-      hl_group = 'Normal',
-      priority = 199,
-    })
-    local header_extmarks =
-      highlight_text(bufnr, ns, hunk, hunk.header_context_col, hunk.header_context, lang)
-    if header_extmarks > 0 then
-      dbg('header %s:%d applied %d extmarks', hunk.filename, hunk.start_line, header_extmarks)
-    end
-  end
-
-  col_offset = col_offset or 1
-
   local extmark_count = 0
   for id, node, metadata in query:iter_captures(trees[1]:root(), code) do
     local capture_name = '@' .. query.captures[id] .. '.' .. lang
     local sr, sc, er, ec = node:range()
 
-    local buf_sr = hunk.start_line + sr
+    local buf_sr = line_map[sr]
-    local buf_er = hunk.start_line + er
+    if buf_sr then
-    local buf_sc = sc + col_offset
+      local buf_er = line_map[er] or buf_sr
-    local buf_ec = ec + col_offset
 
-    local priority = lang == 'diff' and (tonumber(metadata.priority) or 100) or 200
+      local buf_sc = sc + col_offset
+      local buf_ec = ec + col_offset
 
-    pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_sr, buf_sc, {
+      local priority = lang == 'diff' and (tonumber(metadata.priority) or 100) or PRIORITY_SYNTAX
-      end_row = buf_er,
+
-      end_col = buf_ec,
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_sr, buf_sc, {
-      hl_group = capture_name,
+        end_row = buf_er,
-      priority = priority,
+        end_col = buf_ec,
-    })
+        hl_group = capture_name,
-    extmark_count = extmark_count + 1
+        priority = priority,
+      })
+      extmark_count = extmark_count + 1
+      if covered_lines then
+        covered_lines[buf_sr] = true
+      end
+    end
   end
 
   return extmark_count
@@ -176,8 +176,9 @@ end
 ---@param ns integer
 ---@param hunk diffs.Hunk
 ---@param code_lines string[]
+---@param covered_lines? table<integer, true>
 ---@return integer
-local function highlight_vim_syntax(bufnr, ns, hunk, code_lines)
+local function highlight_vim_syntax(bufnr, ns, hunk, code_lines, covered_lines)
   local ft = hunk.ft
   if not ft then
     return 0
@@ -219,9 +220,12 @@ local function highlight_vim_syntax(bufnr, ns, hunk, code_lines)
     pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, span.col_start, {
       end_col = span.col_end,
       hl_group = span.hl_name,
-      priority = 200,
+      priority = PRIORITY_SYNTAX,
     })
     extmark_count = extmark_count + 1
+    if covered_lines then
+      covered_lines[buf_line] = true
+    end
   end
 
   return extmark_count
@@ -248,21 +252,63 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     use_vim = false
   end
 
-  local apply_syntax = use_ts or use_vim
+  ---@type table<integer, true>
-
+  local covered_lines = {}
-  ---@type string[]
-  local code_lines = {}
-  if apply_syntax then
-    for _, line in ipairs(hunk.lines) do
-      table.insert(code_lines, line:sub(2))
-    end
-  end
 
   local extmark_count = 0
   if use_ts then
-    extmark_count = highlight_treesitter(bufnr, ns, hunk, code_lines)
+    ---@type string[]
+    local new_code = {}
+    ---@type table<integer, integer>
+    local new_map = {}
+    ---@type string[]
+    local old_code = {}
+    ---@type table<integer, integer>
+    local old_map = {}
+
+    for i, line in ipairs(hunk.lines) do
+      local prefix = line:sub(1, 1)
+      local stripped = line:sub(2)
+      local buf_line = hunk.start_line + i - 1
+
+      if prefix == '+' then
+        new_map[#new_code] = buf_line
+        table.insert(new_code, stripped)
+      elseif prefix == '-' then
+        old_map[#old_code] = buf_line
+        table.insert(old_code, stripped)
+      else
+        new_map[#new_code] = buf_line
+        table.insert(new_code, stripped)
+        table.insert(old_code, stripped)
+      end
+    end
+
+    extmark_count = highlight_treesitter(bufnr, ns, new_code, hunk.lang, new_map, 1, covered_lines)
+    extmark_count = extmark_count
+      + highlight_treesitter(bufnr, ns, old_code, hunk.lang, old_map, 1, covered_lines)
+
+    if hunk.header_context and hunk.header_context_col then
+      local header_line = hunk.start_line - 1
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, header_line, hunk.header_context_col, {
+        end_col = hunk.header_context_col + #hunk.header_context,
+        hl_group = 'DiffsClear',
+        priority = PRIORITY_CLEAR,
+      })
+      local header_extmarks =
+        highlight_text(bufnr, ns, hunk, hunk.header_context_col, hunk.header_context, hunk.lang)
+      if header_extmarks > 0 then
+        dbg('header %s:%d applied %d extmarks', hunk.filename, hunk.start_line, header_extmarks)
+      end
+      extmark_count = extmark_count + header_extmarks
+    end
   elseif use_vim then
-    extmark_count = highlight_vim_syntax(bufnr, ns, hunk, code_lines)
+    ---@type string[]
+    local code_lines = {}
+    for _, line in ipairs(hunk.lines) do
+      table.insert(code_lines, line:sub(2))
+    end
+    extmark_count = highlight_vim_syntax(bufnr, ns, hunk, code_lines, covered_lines)
   end
 
   if
@@ -271,18 +317,15 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     and #hunk.header_lines > 0
     and opts.highlights.treesitter.enabled
   then
+    ---@type table<integer, integer>
+    local header_map = {}
+    for i = 0, #hunk.header_lines - 1 do
+      header_map[i] = hunk.header_start_line - 1 + i
+    end
     extmark_count = extmark_count
-      + highlight_treesitter(bufnr, ns, {
+      + highlight_treesitter(bufnr, ns, hunk.header_lines, 'diff', header_map, 0)
-        filename = hunk.filename,
-        start_line = hunk.header_start_line - 1,
-        lang = 'diff',
-        lines = hunk.header_lines,
-        header_lines = {},
-      }, hunk.header_lines, 0)
   end
 
-  local syntax_applied = extmark_count > 0
-
   ---@type diffs.IntraChanges?
   local intra = nil
   local intra_cfg = opts.highlights.intra
@@ -334,11 +377,11 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
       })
     end
 
-    if line_len > 1 and syntax_applied then
+    if line_len > 1 and covered_lines[buf_line] then
       pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 1, {
         end_col = line_len,
-        hl_group = 'Normal',
+        hl_group = 'DiffsClear',
-        priority = 198,
+        priority = PRIORITY_CLEAR,
       })
     end
 
@@ -348,7 +391,7 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
         hl_group = line_hl,
         hl_eol = true,
         number_hl_group = opts.highlights.gutter and number_hl or nil,
-        priority = 199,
+        priority = PRIORITY_LINE_BG,
       })
     end
 
@@ -367,7 +410,7 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
         local ok, err = pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, span.col_start, {
           end_col = span.col_end,
           hl_group = char_hl,
-          priority = 201,
+          priority = PRIORITY_CHAR_BG,
         })
         if not ok then
           dbg('char extmark FAILED: %s', err)

lua/diffs/init.lua

@@ -186,6 +186,7 @@ local function compute_highlight_groups()
   local blended_add_text = blend_color(add_fg, bg, 0.7)
   local blended_del_text = blend_color(del_fg, bg, 0.7)
 
+  vim.api.nvim_set_hl(0, 'DiffsClear', { default = true, fg = normal.fg or 0xc0c0c0 })
   vim.api.nvim_set_hl(0, 'DiffsAdd', { default = true, bg = blended_add })
   vim.api.nvim_set_hl(0, 'DiffsDelete', { default = true, bg = blended_del })
   vim.api.nvim_set_hl(0, 'DiffsAddNr', { default = true, fg = add_fg, bg = blended_add })

spec/highlight_spec.lua

@@ -7,8 +7,10 @@ describe('highlight', function()
 
     before_each(function()
       ns = vim.api.nvim_create_namespace('diffs_test')
+      local normal = vim.api.nvim_get_hl(0, { name = 'Normal' })
       local diff_add = vim.api.nvim_get_hl(0, { name = 'DiffAdd' })
       local diff_delete = vim.api.nvim_get_hl(0, { name = 'DiffDelete' })
+      vim.api.nvim_set_hl(0, 'DiffsClear', { fg = normal.fg or 0xc0c0c0 })
       vim.api.nvim_set_hl(0, 'DiffsAdd', { bg = diff_add.bg })
       vim.api.nvim_set_hl(0, 'DiffsDelete', { bg = diff_delete.bg })
     end)
@@ -82,7 +84,7 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
-    it('applies Normal extmarks to clear diff colors', function()
+    it('applies DiffsClear extmarks to clear diff colors', function()
       local bufnr = create_buffer({
         '@@ -1,1 +1,2 @@',
         ' local x = 1',
@@ -99,14 +101,46 @@ describe('highlight', function()
       highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
 
       local extmarks = get_extmarks(bufnr)
-      local has_normal = false
+      local has_clear = false
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].hl_group == 'Normal' then
+        if mark[4] and mark[4].hl_group == 'DiffsClear' then
-          has_normal = true
+          has_clear = true
           break
         end
       end
-      assert.is_true(has_normal)
+      assert.is_true(has_clear)
+      delete_buffer(bufnr)
+    end)
+
+    it('produces treesitter captures on all lines with split parsing', function()
+      local bufnr = create_buffer({
+        '@@ -1,3 +1,3 @@',
+        ' local x = 1',
+        '-local y = 2',
+        '+local y = 3',
+        ' return x',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        lines = { ' local x = 1', '-local y = 2', '+local y = 3', ' return x' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local lines_with_ts = {}
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].hl_group and mark[4].hl_group:match('^@.*%.lua$') then
+          lines_with_ts[mark[2]] = true
+        end
+      end
+      assert.is_true(lines_with_ts[1] ~= nil)
+      assert.is_true(lines_with_ts[2] ~= nil)
+      assert.is_true(lines_with_ts[3] ~= nil)
+      assert.is_true(lines_with_ts[4] ~= nil)
       delete_buffer(bufnr)
     end)
 
@@ -576,7 +610,7 @@ describe('highlight', function()
       local extmarks = get_extmarks(bufnr)
       local has_syntax_hl = false
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].hl_group and mark[4].hl_group ~= 'Normal' then
+        if mark[4] and mark[4].hl_group and mark[4].hl_group ~= 'DiffsClear' then
           has_syntax_hl = true
           break
         end
@@ -610,7 +644,7 @@ describe('highlight', function()
       local extmarks = get_extmarks(bufnr)
       local has_syntax_hl = false
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].hl_group and mark[4].hl_group ~= 'Normal' then
+        if mark[4] and mark[4].hl_group and mark[4].hl_group ~= 'DiffsClear' then
           has_syntax_hl = true
           break
         end
@@ -682,7 +716,7 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
-    it('applies Normal blanking for vim fallback hunks', function()
+    it('applies DiffsClear blanking for vim fallback hunks', function()
       local orig_synID = vim.fn.synID
       local orig_synIDtrans = vim.fn.synIDtrans
       local orig_synIDattr = vim.fn.synIDattr
@@ -722,14 +756,14 @@ describe('highlight', function()
       vim.fn.synIDattr = orig_synIDattr
 
       local extmarks = get_extmarks(bufnr)
-      local has_normal = false
+      local has_clear = false
       for _, mark in ipairs(extmarks) do
-        if mark[4] and mark[4].hl_group == 'Normal' then
+        if mark[4] and mark[4].hl_group == 'DiffsClear' then
-          has_normal = true
+          has_clear = true
           break
         end
       end
-      assert.is_true(has_normal)
+      assert.is_true(has_clear)
       delete_buffer(bufnr)
     end)
 
@@ -765,7 +799,7 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
-    it('line bg priority > Normal priority', function()
+    it('line bg priority > DiffsClear priority', function()
       local bufnr = create_buffer({
         '@@ -1,2 +1,1 @@',
         '-local x = 1',
@@ -787,20 +821,20 @@ describe('highlight', function()
       )
 
       local extmarks = get_extmarks(bufnr)
-      local normal_priority = nil
+      local clear_priority = nil
       local line_bg_priority = nil
       for _, mark in ipairs(extmarks) do
         local d = mark[4]
-        if d and d.hl_group == 'Normal' then
+        if d and d.hl_group == 'DiffsClear' then
-          normal_priority = d.priority
+          clear_priority = d.priority
         end
         if d and (d.hl_group == 'DiffsAdd' or d.hl_group == 'DiffsDelete') then
           line_bg_priority = d.priority
         end
       end
-      assert.is_not_nil(normal_priority)
+      assert.is_not_nil(clear_priority)
       assert.is_not_nil(line_bg_priority)
-      assert.is_true(line_bg_priority > normal_priority)
+      assert.is_true(line_bg_priority > clear_priority)
       delete_buffer(bufnr)
     end)
 
@@ -960,7 +994,7 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
-    it('enforces priority order: Normal < line bg < syntax < char bg', function()
+    it('enforces priority order: DiffsClear < syntax < line bg < char bg', function()
       vim.api.nvim_set_hl(0, 'DiffsAddText', { bg = 0x00FF00 })
       vim.api.nvim_set_hl(0, 'DiffsDeleteText', { bg = 0xFF0000 })
 
@@ -990,12 +1024,12 @@ describe('highlight', function()
       )
 
       local extmarks = get_extmarks(bufnr)
-      local priorities = { normal = {}, line_bg = {}, syntax = {}, char_bg = {} }
+      local priorities = { clear = {}, line_bg = {}, syntax = {}, char_bg = {} }
       for _, mark in ipairs(extmarks) do
         local d = mark[4]
         if d then
-          if d.hl_group == 'Normal' then
+          if d.hl_group == 'DiffsClear' then
-            table.insert(priorities.normal, d.priority)
+            table.insert(priorities.clear, d.priority)
           elseif d.hl_group == 'DiffsAdd' or d.hl_group == 'DiffsDelete' then
             table.insert(priorities.line_bg, d.priority)
           elseif d.hl_group == 'DiffsAddText' or d.hl_group == 'DiffsDeleteText' then
@@ -1006,19 +1040,19 @@ describe('highlight', function()
         end
       end
 
-      assert.is_true(#priorities.normal > 0)
+      assert.is_true(#priorities.clear > 0)
       assert.is_true(#priorities.line_bg > 0)
       assert.is_true(#priorities.syntax > 0)
       assert.is_true(#priorities.char_bg > 0)
 
-      local max_normal = math.max(unpack(priorities.normal))
+      local max_clear = math.max(unpack(priorities.clear))
       local min_line_bg = math.min(unpack(priorities.line_bg))
       local min_syntax = math.min(unpack(priorities.syntax))
       local min_char_bg = math.min(unpack(priorities.char_bg))
 
-      assert.is_true(max_normal < min_line_bg)
+      assert.is_true(max_clear < min_syntax)
-      assert.is_true(min_line_bg < min_syntax)
+      assert.is_true(min_syntax < min_line_bg)
-      assert.is_true(min_syntax < min_char_bg)
+      assert.is_true(min_line_bg < min_char_bg)
       delete_buffer(bufnr)
     end)
   end)
@@ -1214,7 +1248,7 @@ describe('highlight', function()
       }
     end
 
-    it('uses priority 200 for code languages', function()
+    it('uses priority 199 for code languages', function()
       local bufnr = create_buffer({
         '@@ -1,1 +1,2 @@',
         ' local x = 1',
@@ -1231,16 +1265,16 @@ describe('highlight', function()
       highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
 
       local extmarks = get_extmarks(bufnr)
-      local has_priority_200 = false
+      local has_priority_199 = false
       for _, mark in ipairs(extmarks) do
         if mark[4] and mark[4].hl_group and mark[4].hl_group:match('^@.*%.lua$') then
-          if mark[4].priority == 200 then
+          if mark[4].priority == 199 then
-            has_priority_200 = true
+            has_priority_199 = true
             break
           end
         end
       end
-      assert.is_true(has_priority_200)
+      assert.is_true(has_priority_199)
       delete_buffer(bufnr)
     end)
 
@@ -1278,7 +1312,7 @@ describe('highlight', function()
       end
       assert.is_true(#diff_extmark_priorities > 0)
       for _, priority in ipairs(diff_extmark_priorities) do
-        assert.is_true(priority < 200)
+        assert.is_true(priority < 199)
       end
       delete_buffer(bufnr)
     end)