fix(highlight): support combined diff format for unmerged files

Problem: fugitive shows combined diffs (@@@ headers, 2-char prefixes)
for unmerged (UU) files. The parser and highlight pipeline assumed
unified diff format (@@, 1-char prefix), causing broken prefix
concealment, missing background colors on ` +`/`+ ` lines, and no
treesitter highlights due to garbage prefix chars in code arrays.

Solution: detect prefix width from the number of leading @ signs in
hunk headers. Propagate prefix_width through the parser (new field on
diffs.Hunk) and highlight pipeline (prefix stripping, col_offset,
concealment, line classification). Add U to filename pattern for
unmerged file detection. Skip intra-line diffing for combined diffs
since the 2-char prefix semantics don't produce meaningful change
groups.

lua/diffs/highlight.lua

@@ -239,13 +239,14 @@ local function highlight_vim_syntax(
   pcall(vim.api.nvim_buf_delete, scratch, { force = true })
 
   local hunk_line_count = #hunk.lines
+  local col_off = (hunk.prefix_width or 1) - 1
   local extmark_count = 0
   for _, span in ipairs(spans) do
     local adj = span.line - leading_offset
     if adj >= 1 and adj <= hunk_line_count then
       local buf_line = hunk.start_line + adj - 1
-      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, span.col_start, {
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, span.col_start + col_off, {
-        end_col = span.col_end,
+        end_col = span.col_end + col_off,
         hl_group = span.hl_name,
         priority = priorities.syntax,
       })
@@ -265,6 +266,7 @@ end
 ---@param opts diffs.HunkOpts
 function M.highlight_hunk(bufnr, ns, hunk, opts)
   local p = opts.highlights.priorities
+  local pw = hunk.prefix_width or 1
   local use_ts = hunk.lang and opts.highlights.treesitter.enabled
   local use_vim = not use_ts and hunk.ft and opts.highlights.vim.enabled
 
@@ -296,14 +298,16 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     local old_map = {}
 
     for i, line in ipairs(hunk.lines) do
-      local prefix = line:sub(1, 1)
+      local prefix = line:sub(1, pw)
-      local stripped = line:sub(2)
+      local stripped = line:sub(pw + 1)
       local buf_line = hunk.start_line + i - 1
+      local has_add = prefix:find('+', 1, true) ~= nil
+      local has_del = prefix:find('-', 1, true) ~= nil
 
-      if prefix == '+' then
+      if has_add and not has_del then
         new_map[#new_code] = buf_line
         table.insert(new_code, stripped)
-      elseif prefix == '-' then
+      elseif has_del and not has_add then
         old_map[#old_code] = buf_line
         table.insert(old_code, stripped)
       else
@@ -314,9 +318,9 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     end
 
     extmark_count =
-      highlight_treesitter(bufnr, ns, new_code, hunk.lang, new_map, 1, covered_lines, p)
+      highlight_treesitter(bufnr, ns, new_code, hunk.lang, new_map, pw, covered_lines, p)
     extmark_count = extmark_count
-      + highlight_treesitter(bufnr, ns, old_code, hunk.lang, old_map, 1, covered_lines, p)
+      + highlight_treesitter(bufnr, ns, old_code, hunk.lang, old_map, pw, covered_lines, p)
 
     if hunk.header_context and hunk.header_context_col then
       local header_line = hunk.start_line - 1
@@ -344,7 +348,7 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     ---@type string[]
     local code_lines = {}
     for _, line in ipairs(hunk.lines) do
-      table.insert(code_lines, line:sub(2))
+      table.insert(code_lines, line:sub(pw + 1))
     end
     extmark_count = highlight_vim_syntax(bufnr, ns, hunk, code_lines, covered_lines, 0, p)
   end
@@ -367,7 +371,7 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
   ---@type diffs.IntraChanges?
   local intra = nil
   local intra_cfg = opts.highlights.intra
-  if intra_cfg and intra_cfg.enabled and #hunk.lines <= intra_cfg.max_lines then
+  if intra_cfg and intra_cfg.enabled and pw == 1 and #hunk.lines <= intra_cfg.max_lines then
     dbg('computing intra for hunk %s:%d (%d lines)', hunk.filename, hunk.start_line, #hunk.lines)
     intra = diff.compute_intra_hunks(hunk.lines, intra_cfg.algorithm)
     if intra then
@@ -401,22 +405,23 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
   for i, line in ipairs(hunk.lines) do
     local buf_line = hunk.start_line + i - 1
     local line_len = #line
-    local prefix = line:sub(1, 1)
+    local prefix = line:sub(1, pw)
-
+    local has_add = prefix:find('+', 1, true) ~= nil
-    local is_diff_line = prefix == '+' or prefix == '-'
+    local has_del = prefix:find('-', 1, true) ~= nil
-    local line_hl = is_diff_line and (prefix == '+' and 'DiffsAdd' or 'DiffsDelete') or nil
+    local is_diff_line = has_add or has_del
-    local number_hl = is_diff_line and (prefix == '+' and 'DiffsAddNr' or 'DiffsDeleteNr') or nil
+    local line_hl = is_diff_line and (has_add and 'DiffsAdd' or 'DiffsDelete') or nil
+    local number_hl = is_diff_line and (has_add and 'DiffsAddNr' or 'DiffsDeleteNr') or nil
 
     if opts.hide_prefix then
       local virt_hl = (opts.highlights.background and line_hl) or nil
       pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
-        virt_text = { { ' ', virt_hl } },
+        virt_text = { { string.rep(' ', pw), virt_hl } },
         virt_text_pos = 'overlay',
       })
     end
 
-    if line_len > 1 and covered_lines[buf_line] then
+    if line_len > pw and covered_lines[buf_line] then
-      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 1, {
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, pw, {
         end_col = line_len,
         hl_group = 'DiffsClear',
         priority = p.clear,
@@ -439,7 +444,7 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     end
 
     if char_spans_by_line[i] then
-      local char_hl = prefix == '+' and 'DiffsAddText' or 'DiffsDeleteText'
+      local char_hl = has_add and 'DiffsAddText' or 'DiffsDeleteText'
       for _, span in ipairs(char_spans_by_line[i]) do
         dbg(
           'char extmark: line=%d buf_line=%d col=%d..%d hl=%s text="%s"',

lua/diffs/parser.lua

@@ -12,6 +12,7 @@
 ---@field file_old_count integer?
 ---@field file_new_start integer?
 ---@field file_new_count integer?
+---@field prefix_width integer
 ---@field repo_root string?
 
 local M = {}
@@ -133,6 +134,8 @@ function M.parse_buffer(bufnr)
   local hunk_lines = {}
   ---@type integer?
   local hunk_count = nil
+  ---@type integer
+  local hunk_prefix_width = 1
   ---@type integer?
   local header_start = nil
   ---@type string[]
@@ -156,6 +159,7 @@ function M.parse_buffer(bufnr)
         header_context = hunk_header_context,
         header_context_col = hunk_header_context_col,
         lines = hunk_lines,
+        prefix_width = hunk_prefix_width,
         file_old_start = file_old_start,
         file_old_count = file_old_count,
         file_new_start = file_new_start,
@@ -179,7 +183,7 @@ function M.parse_buffer(bufnr)
   end
 
   for i, line in ipairs(lines) do
-    local filename = line:match('^[MADRC%?!]%s+(.+)$') or line:match('^diff %-%-git a/.+ b/(.+)$')
+    local filename = line:match('^[MADRCU%?!]%s+(.+)$') or line:match('^diff %-%-git a/.+ b/(.+)$')
     if filename then
       flush_hunk()
       current_filename = filename
@@ -191,11 +195,15 @@ function M.parse_buffer(bufnr)
         dbg('file: %s -> ft: %s (no ts parser)', filename, current_ft)
       end
       hunk_count = 0
+      hunk_prefix_width = 1
       header_start = i
       header_lines = {}
-    elseif line:match('^@@.-@@') then
+    elseif line:match('^@@+') then
       flush_hunk()
       hunk_start = i
+      local at_prefix = line:match('^(@@+)')
+      hunk_prefix_width = #at_prefix - 1
+      if #at_prefix == 2 then
         local hs, hc, hs2, hc2 = line:match('^@@ %-(%d+),?(%d*) %+(%d+),?(%d*) @@')
         if hs then
           file_old_start = tonumber(hs)
@@ -203,10 +211,17 @@ function M.parse_buffer(bufnr)
           file_new_start = tonumber(hs2)
           file_new_count = tonumber(hc2) or 1
         end
-      local prefix, context = line:match('^(@@.-@@%s*)(.*)')
+      else
+        local hs2, hc2 = line:match('%+(%d+),?(%d*) @@')
+        if hs2 then
+          file_new_start = tonumber(hs2)
+          file_new_count = tonumber(hc2) or 1
+        end
+      end
+      local at_end, context = line:match('^(@@+.-@@+%s*)(.*)')
       if context and context ~= '' then
         hunk_header_context = context
-        hunk_header_context_col = #prefix
+        hunk_header_context_col = #at_end
       end
       if hunk_count then
         hunk_count = hunk_count + 1

spec/highlight_spec.lua

@@ -830,6 +830,114 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
+    it('applies DiffsAdd background to combined diff lines with + in prefix', function()
+      local bufnr = create_buffer({
+        '@@@ -1,3 -1,5 +1,9 @@@',
+        '  local M = {}',
+        '++<<<<<<< HEAD',
+        ' +  return 1',
+        '++=======',
+        '+   return 2',
+        '++>>>>>>> feature',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        prefix_width = 2,
+        lines = {
+          '  local M = {}',
+          '++<<<<<<< HEAD',
+          ' +  return 1',
+          '++=======',
+          '+   return 2',
+          '++>>>>>>> feature',
+        },
+      }
+
+      highlight.highlight_hunk(
+        bufnr,
+        ns,
+        hunk,
+        default_opts({ highlights = { background = true } })
+      )
+
+      local extmarks = get_extmarks(bufnr)
+      local add_lines = {}
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].hl_group == 'DiffsAdd' then
+          add_lines[mark[2]] = true
+        end
+      end
+      assert.is_nil(add_lines[0])
+      assert.is_nil(add_lines[1])
+      assert.is_true(add_lines[2] ~= nil)
+      assert.is_true(add_lines[3] ~= nil)
+      assert.is_true(add_lines[4] ~= nil)
+      assert.is_true(add_lines[5] ~= nil)
+      assert.is_true(add_lines[6] ~= nil)
+      delete_buffer(bufnr)
+    end)
+
+    it('conceals 2-char prefix for combined diffs', function()
+      local bufnr = create_buffer({
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '++<<<<<<< HEAD',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        prefix_width = 2,
+        lines = { '  local M = {}', '++<<<<<<< HEAD' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts({ hide_prefix = true }))
+
+      local extmarks = get_extmarks(bufnr)
+      local overlay_count = 0
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].virt_text_pos == 'overlay' then
+          overlay_count = overlay_count + 1
+          assert.are.equal('  ', mark[4].virt_text[1][1])
+        end
+      end
+      assert.are.equal(2, overlay_count)
+      delete_buffer(bufnr)
+    end)
+
+    it('produces treesitter captures on combined diff content lines', function()
+      local bufnr = create_buffer({
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        ' +local x = 1',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        prefix_width = 2,
+        lines = { '  local M = {}', ' +local x = 1' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local has_ts = false
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].hl_group and mark[4].hl_group:match('^@.*%.lua$') then
+          has_ts = true
+          break
+        end
+      end
+      assert.is_true(has_ts)
+      delete_buffer(bufnr)
+    end)
+
     it('filters @spell and @nospell captures from injections', function()
       local bufnr = create_buffer({
         '@@ -1,1 +1,2 @@',

spec/parser_spec.lua

@@ -425,6 +425,81 @@ describe('parser', function()
       delete_buffer(bufnr)
     end)
 
+    it('recognizes U prefix for unmerged files', function()
+      local bufnr = create_buffer({
+        'U merge_me.lua',
+        '@@@ -1,3 -1,5 +1,9 @@@',
+        '  local M = {}',
+        '++<<<<<<< HEAD',
+        ' +  return 1',
+        '++=======',
+        '+   return 2',
+        '++>>>>>>> feature',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('merge_me.lua', hunks[1].filename)
+      assert.are.equal('lua', hunks[1].ft)
+      delete_buffer(bufnr)
+    end)
+
+    it('sets prefix_width from @@@ combined diff header', function()
+      local bufnr = create_buffer({
+        'U test.lua',
+        '@@@ -1,3 -1,5 +1,9 @@@',
+        '  local M = {}',
+        '++<<<<<<< HEAD',
+        ' +  return 1',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal(2, hunks[1].prefix_width)
+      delete_buffer(bufnr)
+    end)
+
+    it('sets prefix_width 1 for standard @@ unified diff', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal(1, hunks[1].prefix_width)
+      delete_buffer(bufnr)
+    end)
+
+    it('extracts new range from combined diff header', function()
+      local bufnr = create_buffer({
+        'U test.lua',
+        '@@@ -1,3 -1,5 +1,9 @@@',
+        '  local M = {}',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal(1, hunks[1].file_new_start)
+      assert.are.equal(9, hunks[1].file_new_count)
+      delete_buffer(bufnr)
+    end)
+
+    it('extracts header context from combined diff header', function()
+      local bufnr = create_buffer({
+        'U test.lua',
+        '@@@ -1,3 -1,5 +1,9 @@@ function M.greet()',
+        '  local M = {}',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+
+      assert.are.equal(1, #hunks)
+      assert.are.equal('function M.greet()', hunks[1].header_context)
+      delete_buffer(bufnr)
+    end)
+
     it('stores repo_root on hunk when available', function()
       local bufnr = create_buffer({
         'M lua/test.lua',