feat(highlight): support combined diff highlighting

Problem: Combined diffs (`pw > 1`) had no DiffsClear on headers
or `@@@`, no per-char prefix coloring, native TS junk on body
prefixes, and `force_high_priority` flattened all diff captures
to p199 making `@punctuation.special` (cyan) win over
`@diff.minus`/`@diff.plus` (red/green) on `---`/`+++` lines. The
`index` keyword and result hash after `..` were also uncolored.

Solution: Extend `qw > 0` guards to `qw > 0 or pw > 1` for
headers, `@@@`, and body prefixes. Add per-char `@diff.plus` /
`@diff.minus` at p199 on body prefix columns. Apply
`@attribute.diff` on `@@@` lines. Preserve relative priority
offsets in `highlight_treesitter` via `priorities.syntax +
meta_prio - 100`. Add manual `@keyword.diff` and `@constant.diff`
captures on combined `index` lines.

lua/diffs/highlight.lua

@@ -74,6 +74,7 @@ end
 ---@param col_offset integer
 ---@param covered_lines? table<integer, true>
 ---@param priorities diffs.PrioritiesConfig
+---@param force_high_priority? boolean
 ---@return integer
 local function highlight_treesitter(
   bufnr,
@@ -83,7 +84,8 @@ local function highlight_treesitter(
   line_map,
   col_offset,
   covered_lines,
-  priorities
+  priorities,
+  force_high_priority
 )
   local code = table.concat(code_lines, '\n')
   if code == '' then
@@ -123,8 +125,9 @@ local function highlight_treesitter(
           local buf_sc = sc + col_offset
           local buf_ec = ec + col_offset
 
+          local meta_prio = tonumber(metadata.priority) or 100
           local priority = tree_lang == 'diff'
-              and (col_offset > 0 and priorities.syntax or (tonumber(metadata.priority) or 100))
+              and ((col_offset > 0 or force_high_priority) and (priorities.syntax + meta_prio - 100) or meta_prio)
             or priorities.syntax
 
           pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_sr, buf_sc, {
@@ -367,14 +370,14 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
       header_map[i] = hunk.header_start_line - 1 + i
     end
     extmark_count = extmark_count
-      + highlight_treesitter(bufnr, ns, hunk.header_lines, 'diff', header_map, qw, nil, p)
+      + highlight_treesitter(bufnr, ns, hunk.header_lines, 'diff', header_map, qw, nil, p, qw > 0 or pw > 1)
   end
 
   local at_raw_line
-  if qw > 0 and opts.highlights.treesitter.enabled then
+  if (qw > 0 or pw > 1) and opts.highlights.treesitter.enabled then
     local at_buf_line = hunk.start_line - 1
     at_raw_line = vim.api.nvim_buf_get_lines(bufnr, at_buf_line, at_buf_line + 1, false)[1]
-    if at_raw_line then
+    if qw > 0 and at_raw_line then
       local at_logical = at_raw_line:sub(qw + 1)
       local at_map = { [0] = at_buf_line }
       extmark_count = extmark_count
@@ -417,7 +420,7 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
   end
 
   if
-    qw > 0
+    (qw > 0 or pw > 1)
     and hunk.header_start_line
     and hunk.header_lines
     and #hunk.header_lines > 0
@@ -430,16 +433,46 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
         hl_group = 'DiffsClear',
         priority = p.clear,
       })
+
+      if pw > 1 then
+        local hline = hunk.header_lines[i + 1]
+        if hline:match('^index ') then
+          pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, qw, {
+            end_col = qw + 5,
+            hl_group = '@keyword.diff',
+            priority = p.syntax,
+          })
+          local dot_pos = hline:find('%.%.', 1, false)
+          if dot_pos then
+            local rest = hline:sub(dot_pos + 2)
+            local hash = rest:match('^(%x+)')
+            if hash then
+              pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, qw + dot_pos + 1, {
+                end_col = qw + dot_pos + 1 + #hash,
+                hl_group = '@constant.diff',
+                priority = p.syntax,
+              })
+            end
+          end
+        end
+      end
     end
   end
 
-  if qw > 0 and at_raw_line then
+  if (qw > 0 or pw > 1) and at_raw_line then
     local at_buf_line = hunk.start_line - 1
     pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, at_buf_line, 0, {
       end_col = #at_raw_line,
       hl_group = 'DiffsClear',
       priority = p.clear,
     })
+    if pw > 1 and opts.highlights.treesitter.enabled then
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, at_buf_line, qw, {
+        end_col = #at_raw_line,
+        hl_group = '@attribute.diff',
+        priority = p.syntax,
+      })
+    end
   end
 
   if use_ts and hunk.header_context and hunk.header_context_col then
@@ -484,6 +517,25 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
         hl_group = 'DiffsClear',
         priority = p.clear,
       })
+    elseif pw > 1 then
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
+        end_col = pw,
+        hl_group = 'DiffsClear',
+        priority = p.clear,
+      })
+    end
+
+    if pw > 1 then
+      for ci = 0, pw - 1 do
+        local ch = line:sub(ci + 1, ci + 1)
+        if ch == '+' or ch == '-' then
+          pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, ci + qw, {
+            end_col = ci + qw + 1,
+            hl_group = ch == '+' and '@diff.plus' or '@diff.minus',
+            priority = p.syntax,
+          })
+        end
+      end
     end
 
     if line_len > pw and covered_lines[buf_line] then

spec/highlight_spec.lua

@@ -1064,11 +1064,16 @@ describe('highlight', function()
       highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
 
       local extmarks = get_extmarks(bufnr)
+      local content_clear_count = 0
       for _, mark in ipairs(extmarks) do
         if mark[4] and mark[4].hl_group == 'DiffsClear' then
-          assert.are.equal(2, mark[3])
+          assert.is_true(mark[3] == 0 or mark[3] == 2, 'DiffsClear at unexpected col ' .. mark[3])
+          if mark[3] == 2 then
+            content_clear_count = content_clear_count + 1
           end
         end
+      end
+      assert.are.equal(2, content_clear_count)
       delete_buffer(bufnr)
     end)
 
@@ -1363,6 +1368,449 @@ describe('highlight', function()
       assert.are.equal(0, header_extmarks)
       delete_buffer(bufnr)
     end)
+
+    it('does not apply DiffsClear to header lines for non-quoted diffs', 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 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)
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if d and d.hl_group == 'DiffsClear' and mark[3] == 0 and mark[2] < 4 then
+          error('unexpected DiffsClear on header row ' .. mark[2] .. ' for non-quoted diff')
+        end
+      end
+      delete_buffer(bufnr)
+    end)
+
+    it('preserves diff grammar treesitter on headers for non-quoted diffs', function()
+      local bufnr = create_buffer({
+        'diff --git a/parser.lua b/parser.lua',
+        '--- a/parser.lua',
+        '+++ b/parser.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+      })
+
+      local hunk = {
+        filename = 'parser.lua',
+        lang = 'lua',
+        start_line = 4,
+        lines = { ' local M = {}', '+local x = 1' },
+        header_start_line = 1,
+        header_lines = {
+          'diff --git a/parser.lua b/parser.lua',
+          '--- a/parser.lua',
+          '+++ b/parser.lua',
+        },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local header_ts_count = 0
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if mark[2] < 3 and d and d.hl_group and d.hl_group:match('^@.*%.diff$') then
+          header_ts_count = header_ts_count + 1
+        end
+      end
+      assert.is_true(header_ts_count > 0, 'expected diff grammar treesitter on header lines')
+      delete_buffer(bufnr)
+    end)
+
+    it('applies syntax extmarks to combined diff body lines', function()
+      local bufnr = create_buffer({
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '+ local x = 1',
+        ' -local y = 2',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 1,
+        lines = { '  local M = {}', '+ local x = 1', ' -local y = 2' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local syntax_on_body = 0
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if mark[2] >= 1 and d and d.hl_group and d.hl_group:match('^@.*%.lua$') then
+          syntax_on_body = syntax_on_body + 1
+        end
+      end
+      assert.is_true(syntax_on_body > 0, 'expected lua treesitter syntax on combined diff body')
+      delete_buffer(bufnr)
+    end)
+
+    it('applies DiffsClear and per-char diff fg to combined diff body prefixes', function()
+      local bufnr = create_buffer({
+        '@@@',
+        '  unchanged',
+        '+ added',
+        ' -removed',
+        '++both',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 1,
+        lines = { '  unchanged', '+ added', ' -removed', '++both' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local prefix_clears = {}
+      local plus_marks = {}
+      local minus_marks = {}
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if mark[2] >= 1 and d then
+          if d.hl_group == 'DiffsClear' and mark[3] == 0 and d.end_col == 2 then
+            prefix_clears[mark[2]] = true
+          end
+          if d.hl_group == '@diff.plus' and d.priority == 199 then
+            if not plus_marks[mark[2]] then
+              plus_marks[mark[2]] = {}
+            end
+            table.insert(plus_marks[mark[2]], mark[3])
+          end
+          if d.hl_group == '@diff.minus' and d.priority == 199 then
+            if not minus_marks[mark[2]] then
+              minus_marks[mark[2]] = {}
+            end
+            table.insert(minus_marks[mark[2]], mark[3])
+          end
+        end
+      end
+
+      assert.is_true(prefix_clears[1] ~= nil, 'DiffsClear on context prefix')
+      assert.is_true(prefix_clears[2] ~= nil, 'DiffsClear on add prefix')
+      assert.is_true(prefix_clears[3] ~= nil, 'DiffsClear on del prefix')
+      assert.is_true(prefix_clears[4] ~= nil, 'DiffsClear on both-add prefix')
+
+      assert.is_true(plus_marks[2] ~= nil, '@diff.plus on + in "+ added"')
+      assert.are.equal(0, plus_marks[2][1])
+
+      assert.is_true(minus_marks[3] ~= nil, '@diff.minus on - in " -removed"')
+      assert.are.equal(1, minus_marks[3][1])
+
+      assert.is_true(plus_marks[4] ~= nil, '@diff.plus on ++ in "++both"')
+      assert.are.equal(2, #plus_marks[4])
+
+      assert.is_nil(plus_marks[1], 'no @diff.plus on context "  unchanged"')
+      assert.is_nil(minus_marks[1], 'no @diff.minus on context "  unchanged"')
+      delete_buffer(bufnr)
+    end)
+
+    it('applies DiffsClear to headers for combined diffs', function()
+      local bufnr = create_buffer({
+        'diff --combined lua/merge/target.lua',
+        'index abc1234,def5678..ghi9012',
+        '--- a/lua/merge/target.lua',
+        '+++ b/lua/merge/target.lua',
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '+ local x = 1',
+      })
+
+      local hunk = {
+        filename = 'lua/merge/target.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 5,
+        lines = { '  local M = {}', '+ local x = 1' },
+        header_start_line = 1,
+        header_lines = {
+          'diff --combined lua/merge/target.lua',
+          'index abc1234,def5678..ghi9012',
+          '--- a/lua/merge/target.lua',
+          '+++ b/lua/merge/target.lua',
+        },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local clear_lines = {}
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if d and d.hl_group == 'DiffsClear' and mark[3] == 0 and mark[2] < 4 then
+          clear_lines[mark[2]] = true
+        end
+      end
+      assert.is_true(clear_lines[0] ~= nil, 'DiffsClear on diff --combined line')
+      assert.is_true(clear_lines[1] ~= nil, 'DiffsClear on index line')
+      assert.is_true(clear_lines[2] ~= nil, 'DiffsClear on --- line')
+      assert.is_true(clear_lines[3] ~= nil, 'DiffsClear on +++ line')
+      delete_buffer(bufnr)
+    end)
+
+    it('applies @attribute.diff at syntax priority to @@@ line for combined diffs', function()
+      local bufnr = create_buffer({
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '+ local x = 1',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 1,
+        lines = { '  local M = {}', '+ local x = 1' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local has_attr = false
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if mark[2] == 0 and d and d.hl_group == '@attribute.diff' and (d.priority or 0) >= 199 then
+          has_attr = true
+        end
+      end
+      assert.is_true(has_attr, '@attribute.diff at p>=199 on @@@ line')
+      delete_buffer(bufnr)
+    end)
+
+    it('applies DiffsClear to @@@ line for combined diffs', function()
+      local bufnr = create_buffer({
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '+ local x = 1',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 1,
+        lines = { '  local M = {}', '+ local x = 1' },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local has_at_clear = false
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if mark[2] == 0 and d and d.hl_group == 'DiffsClear' and mark[3] == 0 then
+          has_at_clear = true
+        end
+      end
+      assert.is_true(has_at_clear, 'DiffsClear on @@@ line')
+      delete_buffer(bufnr)
+    end)
+
+    it('applies header diff grammar at syntax priority for combined diffs', function()
+      local bufnr = create_buffer({
+        'diff --combined lua/merge/target.lua',
+        'index abc1234,def5678..ghi9012',
+        '--- a/lua/merge/target.lua',
+        '+++ b/lua/merge/target.lua',
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '+ local x = 1',
+      })
+
+      local hunk = {
+        filename = 'lua/merge/target.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 5,
+        lines = { '  local M = {}', '+ local x = 1' },
+        header_start_line = 1,
+        header_lines = {
+          'diff --combined lua/merge/target.lua',
+          'index abc1234,def5678..ghi9012',
+          '--- a/lua/merge/target.lua',
+          '+++ b/lua/merge/target.lua',
+        },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local high_prio_diff = {}
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if mark[2] < 4 and d and d.hl_group and d.hl_group:match('^@.*%.diff$') and (d.priority or 0) >= 199 then
+          high_prio_diff[mark[2]] = true
+        end
+      end
+      assert.is_true(high_prio_diff[2] ~= nil, 'diff grammar at p>=199 on --- line')
+      assert.is_true(high_prio_diff[3] ~= nil, 'diff grammar at p>=199 on +++ line')
+      delete_buffer(bufnr)
+    end)
+
+    it('@diff.minus wins over @punctuation.special on combined diff headers', function()
+      local bufnr = create_buffer({
+        'diff --combined lua/merge/target.lua',
+        'index abc1234,def5678..ghi9012',
+        '--- a/lua/merge/target.lua',
+        '+++ b/lua/merge/target.lua',
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '+ local x = 1',
+      })
+
+      local hunk = {
+        filename = 'lua/merge/target.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 5,
+        lines = { '  local M = {}', '+ local x = 1' },
+        header_start_line = 1,
+        header_lines = {
+          'diff --combined lua/merge/target.lua',
+          'index abc1234,def5678..ghi9012',
+          '--- a/lua/merge/target.lua',
+          '+++ b/lua/merge/target.lua',
+        },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local minus_prio, punct_prio_minus = 0, 0
+      local plus_prio, punct_prio_plus = 0, 0
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if d and d.hl_group then
+          if mark[2] == 2 then
+            if d.hl_group == '@diff.minus.diff' then
+              minus_prio = math.max(minus_prio, d.priority or 0)
+            elseif d.hl_group == '@punctuation.special.diff' then
+              punct_prio_minus = math.max(punct_prio_minus, d.priority or 0)
+            end
+          elseif mark[2] == 3 then
+            if d.hl_group == '@diff.plus.diff' then
+              plus_prio = math.max(plus_prio, d.priority or 0)
+            elseif d.hl_group == '@punctuation.special.diff' then
+              punct_prio_plus = math.max(punct_prio_plus, d.priority or 0)
+            end
+          end
+        end
+      end
+      assert.is_true(minus_prio > punct_prio_minus, '@diff.minus.diff should beat @punctuation.special.diff on --- line')
+      assert.is_true(plus_prio > punct_prio_plus, '@diff.plus.diff should beat @punctuation.special.diff on +++ line')
+      delete_buffer(bufnr)
+    end)
+
+    it('applies @keyword.diff on index word for combined diffs', function()
+      local bufnr = create_buffer({
+        'diff --combined lua/merge/target.lua',
+        'index abc1234,def5678..ghi9012',
+        '--- a/lua/merge/target.lua',
+        '+++ b/lua/merge/target.lua',
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '+ local x = 1',
+      })
+
+      local hunk = {
+        filename = 'lua/merge/target.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 5,
+        lines = { '  local M = {}', '+ local x = 1' },
+        header_start_line = 1,
+        header_lines = {
+          'diff --combined lua/merge/target.lua',
+          'index abc1234,def5678..ghi9012',
+          '--- a/lua/merge/target.lua',
+          '+++ b/lua/merge/target.lua',
+        },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local has_keyword = false
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if mark[2] == 1 and d and d.hl_group == '@keyword.diff' and mark[3] == 0 and (d.end_col or 0) == 5 then
+          has_keyword = true
+        end
+      end
+      assert.is_true(has_keyword, '@keyword.diff at row 1, cols 0-5')
+      delete_buffer(bufnr)
+    end)
+
+    it('applies @constant.diff on result hash for combined diffs', function()
+      local bufnr = create_buffer({
+        'diff --combined lua/merge/target.lua',
+        'index abc1234,def5678..ghi9012',
+        '--- a/lua/merge/target.lua',
+        '+++ b/lua/merge/target.lua',
+        '@@@ -1,2 -1,2 +1,3 @@@',
+        '  local M = {}',
+        '+ local x = 1',
+      })
+
+      local hunk = {
+        filename = 'lua/merge/target.lua',
+        lang = 'lua',
+        prefix_width = 2,
+        start_line = 5,
+        lines = { '  local M = {}', '+ local x = 1' },
+        header_start_line = 1,
+        header_lines = {
+          'diff --combined lua/merge/target.lua',
+          'index abc1234,def5678..ghi9012',
+          '--- a/lua/merge/target.lua',
+          '+++ b/lua/merge/target.lua',
+        },
+      }
+
+      highlight.highlight_hunk(bufnr, ns, hunk, default_opts())
+
+      local extmarks = get_extmarks(bufnr)
+      local has_constant = false
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if mark[2] == 1 and d and d.hl_group == '@constant.diff' and (d.priority or 0) >= 199 then
+          has_constant = true
+        end
+      end
+      assert.is_true(has_constant, '@constant.diff on result hash')
+      delete_buffer(bufnr)
+    end)
   end)
 
   describe('extmark priority', function()