fix(highlight): make intra-line bg visible under line backgrounds (#192)

lua/diffs/highlight.lua

@@ -636,10 +636,9 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
       end
 
       if opts.highlights.background and is_diff_line then
-        local bg_end_col = raw_len or (line_len + qw)
         pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
-          end_row = buf_line,
-          end_col = bg_end_col,
+          end_row = buf_line + 1,
+          end_col = 0,
           hl_group = line_hl,
           hl_eol = true,
           priority = p.line_bg,

lua/diffs/init.lua

@@ -437,7 +437,10 @@ local function ensure_cache(bufnr)
   if config.highlights.context.enabled then
     compute_hunk_context(hunks, config.highlights.context.lines)
   end
-  local carried = entry and not entry.pending_clear and carry_forward_highlighted(entry, hunks)
+  local carried = entry
+    and not entry.pending_clear
+    and #entry.hunks == #hunks
+    and carry_forward_highlighted(entry, hunks)
   hunk_cache[bufnr] = {
     hunks = hunks,
     tick = tick,
@@ -514,6 +517,23 @@ local function find_visible_hunks(hunks, toprow, botrow)
   return first, last
 end
 
+---@param bufnr integer
+---@param ns_id integer
+---@param start_row integer
+---@param end_row integer
+local function clear_ns_by_start(bufnr, ns_id, start_row, end_row)
+  local marks = vim.api.nvim_buf_get_extmarks(
+    bufnr,
+    ns_id,
+    { start_row, 0 },
+    { end_row - 1, 2147483647 },
+    {}
+  )
+  for _, m in ipairs(marks) do
+    vim.api.nvim_buf_del_extmark(bufnr, ns_id, m[1])
+  end
+end
+
 local function compute_highlight_groups(is_default)
   local normal = vim.api.nvim_get_hl(0, { name = 'Normal' })
   local diff_add = vim.api.nvim_get_hl(0, { name = 'DiffAdd' })
@@ -945,7 +965,7 @@ local function init()
           if hunk.header_start_line then
             clear_start = hunk.header_start_line - 1
           end
-          vim.api.nvim_buf_clear_namespace(bufnr, ns, clear_start, clear_end)
+          clear_ns_by_start(bufnr, ns, clear_start, clear_end)
           highlight.highlight_hunk(bufnr, ns, hunk, fast_hl_opts)
           entry.highlighted[i] = true
           count = count + 1
@@ -1185,6 +1205,7 @@ M._test = {
   invalidate_cache = invalidate_cache,
   hunks_eq = hunks_eq,
   process_pending_clear = process_pending_clear,
+  clear_ns_by_start = clear_ns_by_start,
   ft_retry_pending = ft_retry_pending,
   compute_hunk_context = compute_hunk_context,
   compute_highlight_groups = compute_highlight_groups,

spec/decoration_provider_spec.lua

@@ -165,7 +165,7 @@ describe('decoration_provider', function()
   end)
 
   describe('hunk stability', function()
-    it('carries forward highlighted for stable hunks on section expansion', function()
+    it('forces full clear on section expansion (hunk count changed)', function()
       local bufnr = create_buffer({
         'M test.lua',
         '@@ -1,2 +1,2 @@',
@@ -193,14 +193,12 @@ describe('decoration_provider', function()
 
       local updated = diffs._test.hunk_cache[bufnr]
       assert.are.equal(3, #updated.hunks)
-      assert.is_true(updated.highlighted[1] == true)
-      assert.is_nil(updated.highlighted[2])
-      assert.is_true(updated.highlighted[3] == true)
-      assert.is_false(updated.pending_clear)
+      assert.are.same({}, updated.highlighted)
+      assert.is_true(updated.pending_clear)
       delete_buffer(bufnr)
     end)
 
-    it('carries forward highlighted for stable hunks on section collapse', function()
+    it('forces full clear on section collapse (stale extmarks from removed hunks)', function()
       local bufnr = create_buffer({
         'M test.lua',
         '@@ -1,2 +1,2 @@',
@@ -227,9 +225,8 @@ describe('decoration_provider', function()
 
       local updated = diffs._test.hunk_cache[bufnr]
       assert.are.equal(2, #updated.hunks)
-      assert.is_true(updated.highlighted[1] == true)
-      assert.is_true(updated.highlighted[2] == true)
-      assert.is_false(updated.pending_clear)
+      assert.are.same({}, updated.highlighted)
+      assert.is_true(updated.pending_clear)
       delete_buffer(bufnr)
     end)
 

spec/highlight_spec.lua

@@ -397,48 +397,98 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
-    it('line bg extmark survives adjacent clear_namespace starting at next row', function()
-      local bufnr = create_buffer({
-        'diff --git a/foo.py b/foo.py',
-        '@@ -1,2 +1,2 @@',
-        '-old',
-        '+new',
-      })
-
-      local hunk = {
-        filename = 'foo.py',
-        header_start_line = 1,
-        start_line = 2,
-        lines = { '-old', '+new' },
-        prefix_width = 1,
-      }
-
-      highlight.highlight_hunk(
-        bufnr,
-        ns,
-        hunk,
-        default_opts({ highlights = { background = true, treesitter = { enabled = false } } })
-      )
+    it(
+      'nvim_buf_clear_namespace kills line bg extmark whose end_row bleeds into cleared range',
+      function()
+        local bufnr = create_buffer({
+          'diff --git a/foo.py b/foo.py',
+          '@@ -1,2 +1,2 @@',
+          '-old',
+          '+new',
+        })
 
-      local last_body_row = hunk.start_line + #hunk.lines - 1
-      vim.api.nvim_buf_clear_namespace(bufnr, ns, last_body_row + 1, last_body_row + 10)
+        local hunk = {
+          filename = 'foo.py',
+          header_start_line = 1,
+          start_line = 2,
+          lines = { '-old', '+new' },
+          prefix_width = 1,
+        }
+
+        highlight.highlight_hunk(
+          bufnr,
+          ns,
+          hunk,
+          default_opts({ highlights = { background = true, treesitter = { enabled = false } } })
+        )
+
+        local last_body_row = hunk.start_line + #hunk.lines - 1
+        vim.api.nvim_buf_clear_namespace(bufnr, ns, last_body_row + 1, last_body_row + 10)
+
+        local marks = vim.api.nvim_buf_get_extmarks(
+          bufnr,
+          ns,
+          { last_body_row, 0 },
+          { last_body_row, -1 },
+          { details = true }
+        )
+        local has_line_bg = false
+        for _, mark in ipairs(marks) do
+          if mark[4] and mark[4].hl_group == 'DiffsAdd' then
+            has_line_bg = true
+          end
+        end
+        assert.is_false(has_line_bg)
+        delete_buffer(bufnr)
+      end
+    )
+
+    it(
+      'clear_ns_by_start preserves line bg extmark whose end_row bleeds past cleared range',
+      function()
+        local bufnr = create_buffer({
+          'diff --git a/foo.py b/foo.py',
+          '@@ -1,2 +1,2 @@',
+          '-old',
+          '+new',
+        })
 
-      local marks = vim.api.nvim_buf_get_extmarks(
-        bufnr,
-        ns,
-        { last_body_row, 0 },
-        { last_body_row, -1 },
-        { details = true }
-      )
-      local has_line_bg = false
-      for _, mark in ipairs(marks) do
-        if mark[4] and mark[4].hl_group == 'DiffsAdd' then
-          has_line_bg = true
+        local hunk = {
+          filename = 'foo.py',
+          header_start_line = 1,
+          start_line = 2,
+          lines = { '-old', '+new' },
+          prefix_width = 1,
+        }
+
+        highlight.highlight_hunk(
+          bufnr,
+          ns,
+          hunk,
+          default_opts({ highlights = { background = true, treesitter = { enabled = false } } })
+        )
+
+        local last_body_row = hunk.start_line + #hunk.lines - 1
+        local clear_ns_by_start = require('diffs')._test.clear_ns_by_start
+        clear_ns_by_start(bufnr, ns, last_body_row + 1, last_body_row + 10)
+
+        local marks = vim.api.nvim_buf_get_extmarks(
+          bufnr,
+          ns,
+          { last_body_row, 0 },
+          { last_body_row, -1 },
+          { details = true }
+        )
+        local has_line_bg = false
+        for _, mark in ipairs(marks) do
+          if mark[4] and mark[4].hl_group == 'DiffsAdd' then
+            has_line_bg = true
+          end
         end
+        assert.is_true(has_line_bg)
+        delete_buffer(bufnr)
       end
-      assert.is_true(has_line_bg)
-      delete_buffer(bufnr)
-    end)
+    )
 
     it('clear range covers last body line of hunk with header', function()
       local bufnr = create_buffer({