feat: treesitter highlighting for diff headers

Apply treesitter highlighting to diff metadata lines (diff --git, index,
---, +++) using the diff language parser. Header info is attached only
to the first hunk of each file to avoid duplicate highlighting.

Based on PR #52 by @phanen with fixes:
- header_lines now only contains diff metadata, not hunk content
- header info attached only to first hunk per file
- removed arbitrary hunk count restriction

lua/diffs/highlight.lua

@@ -57,8 +57,9 @@ end
 ---@param ns integer
 ---@param hunk diffs.Hunk
 ---@param code_lines string[]
+---@param col_offset integer?
 ---@return integer
-local function highlight_treesitter(bufnr, ns, hunk, code_lines)
+local function highlight_treesitter(bufnr, ns, hunk, code_lines, col_offset)
   local lang = hunk.lang
   if not lang then
     return 0
@@ -101,6 +102,8 @@ local function highlight_treesitter(bufnr, ns, hunk, code_lines)
     end
   end
 
+  col_offset = col_offset or 1
+
   local extmark_count = 0
   for id, node, _ in query:iter_captures(trees[1]:root(), code) do
     local capture_name = '@' .. query.captures[id]
@@ -108,8 +111,8 @@ local function highlight_treesitter(bufnr, ns, hunk, code_lines)
 
     local buf_sr = hunk.start_line + sr
     local buf_er = hunk.start_line + er
-    local buf_sc = sc + 1
+    local buf_sc = sc + col_offset
-    local buf_ec = ec + 1
+    local buf_ec = ec + col_offset
 
     pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_sr, buf_sc, {
       end_row = buf_er,
@@ -257,6 +260,22 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     extmark_count = highlight_vim_syntax(bufnr, ns, hunk, code_lines)
   end
 
+  if
+    hunk.header_start_line
+    and hunk.header_lines
+    and #hunk.header_lines > 0
+    and opts.highlights.treesitter.enabled
+  then
+    extmark_count = extmark_count
+      + highlight_treesitter(bufnr, ns, {
+        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
 
   for i, line in ipairs(hunk.lines) do

lua/diffs/parser.lua

@@ -6,6 +6,8 @@
 ---@field header_context string?
 ---@field header_context_col integer?
 ---@field lines string[]
+---@field header_start_line integer?
+---@field header_lines string[]?
 
 local M = {}
 
@@ -58,10 +60,16 @@ function M.parse_buffer(bufnr)
   local hunk_header_context_col = nil
   ---@type string[]
   local hunk_lines = {}
+  ---@type integer?
+  local hunk_count = nil
+  ---@type integer?
+  local header_start = nil
+  ---@type string[]
+  local header_lines = {}
 
   local function flush_hunk()
     if hunk_start and #hunk_lines > 0 and (current_lang or current_ft) then
-      table.insert(hunks, {
+      local hunk = {
         filename = current_filename,
         ft = current_ft,
         lang = current_lang,
@@ -69,7 +77,12 @@ function M.parse_buffer(bufnr)
         header_context = hunk_header_context,
         header_context_col = hunk_header_context_col,
         lines = hunk_lines,
-      })
+      }
+      if hunk_count == 1 and header_start and #header_lines > 0 then
+        hunk.header_start_line = header_start
+        hunk.header_lines = header_lines
+      end
+      table.insert(hunks, hunk)
     end
     hunk_start = nil
     hunk_header_context = nil
@@ -89,6 +102,9 @@ function M.parse_buffer(bufnr)
       elseif current_ft then
         dbg('file: %s -> ft: %s (no ts parser)', filename, current_ft)
       end
+      hunk_count = 0
+      header_start = i
+      header_lines = {}
     elseif line:match('^@@.-@@') then
       flush_hunk()
       hunk_start = i
@@ -97,6 +113,9 @@ function M.parse_buffer(bufnr)
         hunk_header_context = context
         hunk_header_context_col = #prefix
       end
+      if hunk_count then
+        hunk_count = hunk_count + 1
+      end
     elseif hunk_start then
       local prefix = line:sub(1, 1)
       if prefix == ' ' or prefix == '+' or prefix == '-' then
@@ -112,8 +131,12 @@ function M.parse_buffer(bufnr)
         current_filename = nil
         current_ft = nil
         current_lang = nil
+        header_start = nil
       end
     end
+    if header_start and not hunk_start then
+      table.insert(header_lines, line)
+    end
   end
 
   flush_hunk()

spec/highlight_spec.lua

@@ -729,6 +729,176 @@ describe('highlight', function()
     end)
   end)
 
+  describe('diff header highlighting', function()
+    local ns
+
+    before_each(function()
+      ns = vim.api.nvim_create_namespace('diffs_test_header')
+    end)
+
+    local function create_buffer(lines)
+      local bufnr = vim.api.nvim_create_buf(false, true)
+      vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)
+      return bufnr
+    end
+
+    local function delete_buffer(bufnr)
+      if vim.api.nvim_buf_is_valid(bufnr) then
+        vim.api.nvim_buf_delete(bufnr, { force = true })
+      end
+    end
+
+    local function get_extmarks(bufnr)
+      return vim.api.nvim_buf_get_extmarks(bufnr, ns, 0, -1, { details = true })
+    end
+
+    local function default_opts()
+      return {
+        hide_prefix = false,
+        highlights = {
+          background = false,
+          gutter = false,
+          treesitter = { enabled = true, max_lines = 500 },
+          vim = { enabled = false, max_lines = 200 },
+        },
+      }
+    end
+
+    local function has_diff_parser()
+      return pcall(vim.treesitter.language.inspect, 'diff')
+    end
+
+    it('applies treesitter extmarks to diff header lines', function()
+      if not has_diff_parser() then
+        pending('diff treesitter parser not installed')
+        return
+      end
+
+      local bufnr = create_buffer({
+        'diff --git a/parser.lua b/parser.lua',
+        'index 3e8afa0..018159c 100644',
+        '--- a/parser.lua',
+        '+++ b/parser.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+      })
+
+      local hunk = {
+        filename = 'parser.lua',
+        lang = 'lua',
+        start_line = 5,
+        lines = { ' local M = {}', '+local x = 1' },
+        header_start_line = 1,
+        header_lines = {
+          'diff --git a/parser.lua b/parser.lua',
+          'index 3e8afa0..018159c 100644',
+          '--- a/parser.lua',
+          '+++ b/parser.lua',
+        },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local header_extmarks = {}
+      for _, mark in ipairs(extmarks) do
+        if mark[2] < 4 and mark[4] and mark[4].hl_group then
+          table.insert(header_extmarks, mark)
+        end
+      end
+
+      assert.is_true(#header_extmarks > 0)
+
+      local has_function_hl = false
+      local has_keyword_hl = false
+      for _, mark in ipairs(header_extmarks) do
+        local hl = mark[4].hl_group
+        if hl == '@function' or hl == '@function.diff' then
+          has_function_hl = true
+        end
+        if hl == '@keyword' or hl == '@keyword.diff' then
+          has_keyword_hl = true
+        end
+      end
+      assert.is_true(has_function_hl or has_keyword_hl)
+      delete_buffer(bufnr)
+    end)
+
+    it('does not apply header highlights when header_lines missing', function()
+      local bufnr = create_buffer({
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+      })
+
+      local hunk = {
+        filename = 'parser.lua',
+        lang = 'lua',
+        start_line = 1,
+        lines = { ' local M = {}', '+local x = 1' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local header_extmarks = 0
+      for _, mark in ipairs(extmarks) do
+        if mark[2] < 0 and mark[4] and mark[4].hl_group then
+          header_extmarks = header_extmarks + 1
+        end
+      end
+      assert.are.equal(0, header_extmarks)
+      delete_buffer(bufnr)
+    end)
+
+    it('does not apply header highlights when treesitter disabled', function()
+      if not has_diff_parser() then
+        pending('diff treesitter parser not installed')
+        return
+      end
+
+      local bufnr = create_buffer({
+        'diff --git a/parser.lua b/parser.lua',
+        'index 3e8afa0..018159c 100644',
+        '--- a/parser.lua',
+        '+++ b/parser.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+      })
+
+      local hunk = {
+        filename = 'parser.lua',
+        lang = 'lua',
+        start_line = 5,
+        lines = { ' local M = {}', '+local x = 1' },
+        header_start_line = 1,
+        header_lines = {
+          'diff --git a/parser.lua b/parser.lua',
+          'index 3e8afa0..018159c 100644',
+          '--- a/parser.lua',
+          '+++ b/parser.lua',
+        },
+      }
+
+      local opts = default_opts()
+      opts.highlights.treesitter.enabled = false
+
+      highlight.highlight_hunk(bufnr, ns, hunk, opts)
+
+      local extmarks = get_extmarks(bufnr)
+      local header_extmarks = 0
+      for _, mark in ipairs(extmarks) do
+        if mark[2] < 4 and mark[4] and mark[4].hl_group and mark[4].hl_group:match('^@') then
+          header_extmarks = header_extmarks + 1
+        end
+      end
+      assert.are.equal(0, header_extmarks)
+      delete_buffer(bufnr)
+    end)
+  end)
+
   describe('coalesce_syntax_spans', function()
     it('coalesces adjacent chars with same hl group', function()
       local function query_fn(_line, _col)

spec/parser_spec.lua

@@ -215,5 +215,75 @@ describe('parser', function()
       assert.are.equal(1, #hunks[2].lines)
       delete_buffer(bufnr)
     end)
+
+    it('attaches header_lines to first hunk only', function()
+      local bufnr = create_buffer({
+        'diff --git a/parser.lua b/parser.lua',
+        'index 3e8afa0..018159c 100644',
+        '--- a/parser.lua',
+        '+++ b/parser.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+        '@@ -10,2 +11,3 @@',
+        ' function M.foo()',
+        '+  return true',
+        ' end',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(2, #hunks)
+      assert.is_not_nil(hunks[1].header_start_line)
+      assert.is_not_nil(hunks[1].header_lines)
+      assert.are.equal(1, hunks[1].header_start_line)
+      assert.is_nil(hunks[2].header_start_line)
+      assert.is_nil(hunks[2].header_lines)
+      delete_buffer(bufnr)
+    end)
+
+    it('header_lines contains only diff metadata, not hunk content', function()
+      local bufnr = create_buffer({
+        'diff --git a/parser.lua b/parser.lua',
+        'index 3e8afa0..018159c 100644',
+        '--- a/parser.lua',
+        '+++ b/parser.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal(4, #hunks[1].header_lines)
+      assert.are.equal('diff --git a/parser.lua b/parser.lua', hunks[1].header_lines[1])
+      assert.are.equal('index 3e8afa0..018159c 100644', hunks[1].header_lines[2])
+      assert.are.equal('--- a/parser.lua', hunks[1].header_lines[3])
+      assert.are.equal('+++ b/parser.lua', hunks[1].header_lines[4])
+      delete_buffer(bufnr)
+    end)
+
+    it('handles fugitive status format with diff headers', function()
+      local bufnr = create_buffer({
+        'Head: main',
+        'Push: origin/main',
+        '',
+        'Unstaged (1)',
+        'M parser.lua',
+        'diff --git a/parser.lua b/parser.lua',
+        'index 3e8afa0..018159c 100644',
+        '--- a/parser.lua',
+        '+++ b/parser.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal(6, hunks[1].header_start_line)
+      assert.are.equal(4, #hunks[1].header_lines)
+      assert.are.equal('diff --git a/parser.lua b/parser.lua', hunks[1].header_lines[1])
+      delete_buffer(bufnr)
+    end)
   end)
 end)