feat(gitsigns): highlight blame popup with treesitter and intra-line diffs

Problem: gitsigns blame popups show flat `GitSignsAddPreview`/
`GitSignsDeletePreview` highlights with no treesitter syntax or
character-level intra-line diffs.

Solution: add `lua/diffs/gitsigns.lua` which patches `Popup.create`
and `Popup.update` to intercept blame popups, parse hunk sections,
clear gitsigns' own highlights, and apply `highlight_hunk` with
manual `@diff.plus`/`@diff.minus` prefix extmarks. Wired in
`plugin/diffs.lua` with `User GitAttach` lazy-load retry.

lua/diffs/gitsigns.lua

@@ -0,0 +1,183 @@
+local M = {}
+
+local api = vim.api
+local fn = vim.fn
+local dbg = require('diffs.log').dbg
+
+local ns = api.nvim_create_namespace('diffs-gitsigns')
+local gs_popup_ns = api.nvim_create_namespace('gitsigns_popup')
+
+local patched = false
+
+---@param ft string
+---@return string?
+local function get_lang(ft)
+  local lang = vim.treesitter.language.get_lang(ft)
+  if not lang then
+    return nil
+  end
+  local ok = pcall(vim.treesitter.language.inspect, lang)
+  return ok and lang or nil
+end
+
+---@param bufnr integer
+---@param src_filename string
+---@param src_ft string?
+---@param src_lang string?
+---@return diffs.Hunk[]
+function M.parse_blame_hunks(bufnr, src_filename, src_ft, src_lang)
+  local lines = api.nvim_buf_get_lines(bufnr, 0, -1, false)
+  local hunks = {}
+  local hunk_lines = {}
+  local hunk_start = nil
+
+  for i, line in ipairs(lines) do
+    if line:match('^Hunk %d+ of %d+') then
+      if hunk_start and #hunk_lines > 0 then
+        table.insert(hunks, {
+          filename = src_filename,
+          ft = src_ft,
+          lang = src_lang,
+          start_line = hunk_start,
+          prefix_width = 1,
+          quote_width = 0,
+          lines = hunk_lines,
+        })
+      end
+      hunk_lines = {}
+      hunk_start = i
+    elseif hunk_start then
+      if line:match('^%(guessed:') then
+        hunk_start = i
+      else
+        local prefix = line:sub(1, 1)
+        if prefix == ' ' or prefix == '+' or prefix == '-' then
+          if #hunk_lines == 0 then
+            hunk_start = i - 1
+          end
+          table.insert(hunk_lines, line)
+        end
+      end
+    end
+  end
+
+  if hunk_start and #hunk_lines > 0 then
+    table.insert(hunks, {
+      filename = src_filename,
+      ft = src_ft,
+      lang = src_lang,
+      start_line = hunk_start,
+      prefix_width = 1,
+      quote_width = 0,
+      lines = hunk_lines,
+    })
+  end
+
+  return hunks
+end
+
+---@param preview_winid integer
+---@param preview_bufnr integer
+local function on_preview(preview_winid, preview_bufnr)
+  local ok, err = pcall(function()
+    if not api.nvim_buf_is_valid(preview_bufnr) then
+      return
+    end
+    if not api.nvim_win_is_valid(preview_winid) then
+      return
+    end
+
+    local win = api.nvim_get_current_win()
+    if win == preview_winid then
+      win = fn.win_getid(fn.winnr('#'))
+    end
+    if win == -1 or win == 0 or not api.nvim_win_is_valid(win) then
+      return
+    end
+
+    local srcbuf = api.nvim_win_get_buf(win)
+    if not api.nvim_buf_is_loaded(srcbuf) then
+      return
+    end
+
+    local ft = vim.bo[srcbuf].filetype
+    local name = api.nvim_buf_get_name(srcbuf)
+    if not name or name == '' then
+      name = ft and ('a.' .. ft) or 'unknown'
+    end
+    local lang = ft and get_lang(ft) or nil
+
+    local hunks = M.parse_blame_hunks(preview_bufnr, name, ft, lang)
+    if #hunks == 0 then
+      return
+    end
+
+    local diff_start = hunks[1].start_line
+    local last = hunks[#hunks]
+    local diff_end = last.start_line + #last.lines
+
+    api.nvim_buf_clear_namespace(preview_bufnr, gs_popup_ns, diff_start, diff_end)
+    api.nvim_buf_clear_namespace(preview_bufnr, ns, diff_start, diff_end)
+
+    local opts = require('diffs').get_highlight_opts()
+    local highlight = require('diffs.highlight')
+    for _, hunk in ipairs(hunks) do
+      highlight.highlight_hunk(preview_bufnr, ns, hunk, opts)
+      for j, line in ipairs(hunk.lines) do
+        local ch = line:sub(1, 1)
+        if ch == '+' or ch == '-' then
+          pcall(api.nvim_buf_set_extmark, preview_bufnr, ns, hunk.start_line + j - 1, 0, {
+            end_col = 1,
+            hl_group = ch == '+' and '@diff.plus' or '@diff.minus',
+            priority = opts.highlights.priorities.syntax,
+          })
+        end
+      end
+    end
+
+    dbg('gitsigns blame: highlighted %d hunks in popup buf %d', #hunks, preview_bufnr)
+  end)
+  if not ok then
+    dbg('gitsigns blame error: %s', err)
+  end
+end
+
+---@return boolean
+function M.setup()
+  if patched then
+    return true
+  end
+
+  local pop_ok, Popup = pcall(require, 'gitsigns.popup')
+  if not pop_ok or not Popup then
+    return false
+  end
+
+  Popup.create = (function(orig)
+    return function(...)
+      local winid, bufnr = orig(...)
+      on_preview(winid, bufnr)
+      return winid, bufnr
+    end
+  end)(Popup.create)
+
+  Popup.update = (function(orig)
+    return function(winid, bufnr, ...)
+      orig(winid, bufnr, ...)
+      on_preview(winid, bufnr)
+    end
+  end)(Popup.update)
+
+  patched = true
+  dbg('gitsigns popup patched')
+  return true
+end
+
+M._test = {
+  parse_blame_hunks = M.parse_blame_hunks,
+  on_preview = on_preview,
+  ns = ns,
+  gs_popup_ns = gs_popup_ns,
+}
+
+return M

plugin/diffs.lua

@@ -5,6 +5,19 @@ vim.g.loaded_diffs = 1
 
 require('diffs.commands').setup()
 
+local gs_cfg = (vim.g.diffs or {}).gitsigns
+if gs_cfg == true or type(gs_cfg) == 'table' then
+  if not require('diffs.gitsigns').setup() then
+    vim.api.nvim_create_autocmd('User', {
+      pattern = 'GitAttach',
+      once = true,
+      callback = function()
+        require('diffs.gitsigns').setup()
+      end,
+    })
+  end
+end
+
 vim.api.nvim_create_autocmd('FileType', {
   pattern = require('diffs').compute_filetypes(vim.g.diffs or {}),
   callback = function(args)

spec/gitsigns_spec.lua

@@ -0,0 +1,236 @@
+require('spec.helpers')
+local gs = require('diffs.gitsigns')
+
+local function setup_highlight_groups()
+  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 or 0x2e4a3a })
+  vim.api.nvim_set_hl(0, 'DiffsDelete', { bg = diff_delete.bg or 0x4a2e3a })
+  vim.api.nvim_set_hl(0, 'DiffsAddText', { bg = 0x00FF00 })
+  vim.api.nvim_set_hl(0, 'DiffsDeleteText', { bg = 0xFF0000 })
+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 or {})
+  return bufnr
+end
+
+local function delete_buffer(bufnr)
+  if bufnr and vim.api.nvim_buf_is_valid(bufnr) then
+    vim.api.nvim_buf_delete(bufnr, { force = true })
+  end
+end
+
+describe('gitsigns', function()
+  describe('parse_blame_hunks', function()
+    it('parses a single hunk', function()
+      local bufnr = create_buffer({
+        'commit abc1234',
+        'Author: Test User',
+        '',
+        'Hunk 1 of 1',
+        ' local x = 1',
+        '-local y = 2',
+        '+local y = 3',
+        ' local z = 4',
+      })
+      local hunks = gs.parse_blame_hunks(bufnr, 'test.lua', 'lua', 'lua')
+      assert.are.equal(1, #hunks)
+      assert.are.equal('test.lua', hunks[1].filename)
+      assert.are.equal('lua', hunks[1].ft)
+      assert.are.equal('lua', hunks[1].lang)
+      assert.are.equal(1, hunks[1].prefix_width)
+      assert.are.equal(0, hunks[1].quote_width)
+      assert.are.equal(4, #hunks[1].lines)
+      assert.are.equal(4, hunks[1].start_line)
+      assert.are.equal(' local x = 1', hunks[1].lines[1])
+      assert.are.equal('-local y = 2', hunks[1].lines[2])
+      assert.are.equal('+local y = 3', hunks[1].lines[3])
+      delete_buffer(bufnr)
+    end)
+
+    it('parses multiple hunks', function()
+      local bufnr = create_buffer({
+        'commit abc1234',
+        '',
+        'Hunk 1 of 2',
+        '-local a = 1',
+        '+local a = 2',
+        'Hunk 2 of 2',
+        ' local b = 3',
+        '+local c = 4',
+      })
+      local hunks = gs.parse_blame_hunks(bufnr, 'test.lua', 'lua', 'lua')
+      assert.are.equal(2, #hunks)
+      assert.are.equal(2, #hunks[1].lines)
+      assert.are.equal(2, #hunks[2].lines)
+      delete_buffer(bufnr)
+    end)
+
+    it('skips guessed-offset lines', function()
+      local bufnr = create_buffer({
+        'commit abc1234',
+        '',
+        'Hunk 1 of 1',
+        '(guessed: hunk offset may be wrong)',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      local hunks = gs.parse_blame_hunks(bufnr, 'test.lua', 'lua', 'lua')
+      assert.are.equal(1, #hunks)
+      assert.are.equal(2, #hunks[1].lines)
+      assert.are.equal(' local x = 1', hunks[1].lines[1])
+      delete_buffer(bufnr)
+    end)
+
+    it('returns empty table when no hunks present', function()
+      local bufnr = create_buffer({
+        'commit abc1234',
+        'Author: Test User',
+        'Date: 2024-01-01',
+      })
+      local hunks = gs.parse_blame_hunks(bufnr, 'test.lua', 'lua', 'lua')
+      assert.are.equal(0, #hunks)
+      delete_buffer(bufnr)
+    end)
+
+    it('handles hunk with no diff lines after header', function()
+      local bufnr = create_buffer({
+        'Hunk 1 of 1',
+        'some non-diff text',
+      })
+      local hunks = gs.parse_blame_hunks(bufnr, 'test.lua', 'lua', 'lua')
+      assert.are.equal(0, #hunks)
+      delete_buffer(bufnr)
+    end)
+  end)
+
+  describe('on_preview', function()
+    before_each(function()
+      setup_highlight_groups()
+    end)
+
+    it('applies extmarks to popup buffer with diff content', function()
+      local bufnr = create_buffer({
+        'commit abc1234',
+        '',
+        'Hunk 1 of 1',
+        ' local x = 1',
+        '-local y = 2',
+        '+local y = 3',
+      })
+
+      local winid = vim.api.nvim_open_win(bufnr, false, {
+        relative = 'editor',
+        width = 40,
+        height = 10,
+        row = 0,
+        col = 0,
+      })
+
+      gs._test.on_preview(winid, bufnr)
+
+      local extmarks = vim.api.nvim_buf_get_extmarks(bufnr, gs._test.ns, 0, -1, { details = true })
+      assert.is_true(#extmarks > 0)
+
+      vim.api.nvim_win_close(winid, true)
+      delete_buffer(bufnr)
+    end)
+
+    it('clears gitsigns_popup namespace on diff region', function()
+      local bufnr = create_buffer({
+        'commit abc1234',
+        '',
+        'Hunk 1 of 1',
+        ' local x = 1',
+        '+local y = 2',
+      })
+
+      vim.api.nvim_buf_set_extmark(bufnr, gs._test.gs_popup_ns, 3, 0, {
+        end_col = 12,
+        hl_group = 'GitSignsAddPreview',
+      })
+      vim.api.nvim_buf_set_extmark(bufnr, gs._test.gs_popup_ns, 4, 0, {
+        end_col = 12,
+        hl_group = 'GitSignsAddPreview',
+      })
+
+      local winid = vim.api.nvim_open_win(bufnr, false, {
+        relative = 'editor',
+        width = 40,
+        height = 10,
+        row = 0,
+        col = 0,
+      })
+
+      gs._test.on_preview(winid, bufnr)
+
+      local gs_extmarks =
+        vim.api.nvim_buf_get_extmarks(bufnr, gs._test.gs_popup_ns, 0, -1, { details = true })
+      assert.are.equal(0, #gs_extmarks)
+
+      vim.api.nvim_win_close(winid, true)
+      delete_buffer(bufnr)
+    end)
+
+    it('does not error on invalid buffer', function()
+      assert.has_no.errors(function()
+        gs._test.on_preview(0, 99999)
+      end)
+    end)
+  end)
+
+  describe('setup', function()
+    it('returns false when gitsigns.popup is not available', function()
+      local saved = package.loaded['gitsigns.popup']
+      package.loaded['gitsigns.popup'] = nil
+      package.preload['gitsigns.popup'] = nil
+
+      local fresh = loadfile('lua/diffs/gitsigns.lua')()
+      local result = fresh.setup()
+      assert.is_false(result)
+
+      package.loaded['gitsigns.popup'] = saved
+    end)
+
+    it('patches gitsigns.popup when available', function()
+      local create_called = false
+      local update_called = false
+      local mock_popup = {
+        create = function()
+          create_called = true
+          local bufnr = create_buffer({ 'test' })
+          local winid = vim.api.nvim_open_win(bufnr, false, {
+            relative = 'editor',
+            width = 10,
+            height = 1,
+            row = 0,
+            col = 0,
+          })
+          return winid, bufnr
+        end,
+        update = function()
+          update_called = true
+        end,
+      }
+
+      local saved = package.loaded['gitsigns.popup']
+      package.loaded['gitsigns.popup'] = mock_popup
+
+      local fresh = loadfile('lua/diffs/gitsigns.lua')()
+      local result = fresh.setup()
+      assert.is_true(result)
+
+      mock_popup.create()
+      assert.is_true(create_called)
+
+      mock_popup.update(0, 0)
+      assert.is_true(update_called)
+
+      package.loaded['gitsigns.popup'] = saved
+    end)
+  end)
+end)