fix(highlight): use hl_group instead of line_hl_group for diff backgrounds

line_hl_group bg occupies a separate rendering channel from hl_group in
Neovim's extmark system, causing character-level bg-only highlights to be
invisible regardless of priority. Switching to hl_group + hl_eol ensures
all backgrounds compete in the same channel.

Also reorders priorities (Normal 198 < line bg 199 < syntax 200 < char
bg 201), bumps char-level blend alpha from 0.4 to 0.7 for visibility,
and adds debug logging throughout the intra pipeline.

lua/diffs/debug.lua

@@ -0,0 +1,68 @@
+local M = {}
+
+local ns = vim.api.nvim_create_namespace('diffs')
+
+function M.dump()
+  local bufnr = vim.api.nvim_get_current_buf()
+  local marks = vim.api.nvim_buf_get_extmarks(bufnr, ns, 0, -1, { details = true })
+  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+
+  local by_line = {}
+  for _, mark in ipairs(marks) do
+    local id, row, col, details = mark[1], mark[2], mark[3], mark[4]
+    local entry = {
+      id = id,
+      row = row,
+      col = col,
+      end_row = details.end_row,
+      end_col = details.end_col,
+      hl_group = details.hl_group,
+      priority = details.priority,
+      line_hl_group = details.line_hl_group,
+      number_hl_group = details.number_hl_group,
+      virt_text = details.virt_text,
+    }
+    if not by_line[row] then
+      by_line[row] = { text = lines[row + 1] or '', marks = {} }
+    end
+    table.insert(by_line[row].marks, entry)
+  end
+
+  local all_ns_marks = vim.api.nvim_buf_get_extmarks(bufnr, -1, 0, -1, { details = true })
+  local non_diffs = {}
+  for _, mark in ipairs(all_ns_marks) do
+    local details = mark[4]
+    if details.ns_id ~= ns then
+      table.insert(non_diffs, {
+        ns_id = details.ns_id,
+        row = mark[2],
+        col = mark[3],
+        end_row = details.end_row,
+        end_col = details.end_col,
+        hl_group = details.hl_group,
+        priority = details.priority,
+      })
+    end
+  end
+
+  local result = {
+    bufnr = bufnr,
+    buf_name = vim.api.nvim_buf_get_name(bufnr),
+    ns_id = ns,
+    total_diffs_marks = #marks,
+    total_all_marks = #all_ns_marks,
+    non_diffs_marks = non_diffs,
+    lines = by_line,
+  }
+
+  local state_dir = vim.fn.stdpath('state')
+  local path = state_dir .. '/diffs_debug.json'
+  local f = io.open(path, 'w')
+  if f then
+    f:write(vim.json.encode(result))
+    f:close()
+    vim.notify('[diffs.nvim] debug dump: ' .. path, vim.log.levels.INFO)
+  end
+end
+
+return M

lua/diffs/diff.lua

@@ -312,13 +312,28 @@ function M.compute_intra_hunks(hunk_lines, algorithm)
   ---@type diffs.CharSpan[]
   local all_del = {}
 
-  for _, group in ipairs(groups) do
+  dbg(
+    'intra: %d change groups, algorithm=%s, vscode=%s',
+    #groups,
+    algorithm,
+    vscode_handle and 'yes' or 'no'
+  )
+
+  for gi, group in ipairs(groups) do
+    dbg('group %d: %d del lines, %d add lines', gi, #group.del_lines, #group.add_lines)
     local ds, as
     if vscode_handle then
       ds, as = diff_group_vscode(group, vscode_handle)
     else
       ds, as = diff_group_native(group)
     end
+    dbg('group %d result: %d del spans, %d add spans', gi, #ds, #as)
+    for _, s in ipairs(ds) do
+      dbg('  del span: line=%d col=%d..%d', s.line, s.col_start, s.col_end)
+    end
+    for _, s in ipairs(as) do
+      dbg('  add span: line=%d col=%d..%d', s.line, s.col_start, s.col_end)
+    end
     vim.list_extend(all_del, ds)
     vim.list_extend(all_add, as)
   end

lua/diffs/highlight.lua

@@ -287,7 +287,17 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
   local intra = nil
   local intra_cfg = opts.highlights.intra
   if intra_cfg and intra_cfg.enabled 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
+      dbg('intra result: %d add spans, %d del spans', #intra.add_spans, #intra.del_spans)
+    else
+      dbg('intra result: nil (no change groups)')
+    end
+  elseif intra_cfg and not intra_cfg.enabled then
+    dbg('intra disabled by config')
+  elseif intra_cfg and #hunk.lines > intra_cfg.max_lines then
+    dbg('intra skipped: %d lines > %d max', #hunk.lines, intra_cfg.max_lines)
   end
 
   ---@type table<integer, diffs.CharSpan[]>
@@ -324,21 +334,20 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
       })
     end
 
-    if opts.highlights.background and is_diff_line then
-      local extmark_opts = {
-        line_hl_group = line_hl,
-        priority = 198,
-      }
-      if opts.highlights.gutter then
-        extmark_opts.number_hl_group = number_hl
-      end
-      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, extmark_opts)
-    end
-
     if line_len > 1 and syntax_applied then
       pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 1, {
         end_col = line_len,
         hl_group = 'Normal',
+        priority = 198,
+      })
+    end
+
+    if opts.highlights.background and is_diff_line then
+      pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
+        end_col = line_len,
+        hl_group = line_hl,
+        hl_eol = true,
+        number_hl_group = opts.highlights.gutter and number_hl or nil,
         priority = 199,
       })
     end
@@ -346,11 +355,23 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
     if char_spans_by_line[i] then
       local char_hl = prefix == '+' and 'DiffsAddText' or 'DiffsDeleteText'
       for _, span in ipairs(char_spans_by_line[i]) do
-        pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, span.col_start, {
+        dbg(
+          'char extmark: line=%d buf_line=%d col=%d..%d hl=%s text="%s"',
+          i,
+          buf_line,
+          span.col_start,
+          span.col_end,
+          char_hl,
+          line:sub(span.col_start + 1, span.col_end)
+        )
+        local ok, err = pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, span.col_start, {
           end_col = span.col_end,
           hl_group = char_hl,
           priority = 201,
         })
+        if not ok then
+          dbg('char extmark FAILED: %s', err)
+        end
         extmark_count = extmark_count + 1
       end
     end

lua/diffs/init.lua

@@ -183,8 +183,8 @@ local function compute_highlight_groups()
   local blended_add = blend_color(add_bg, bg, 0.4)
   local blended_del = blend_color(del_bg, bg, 0.4)
 
-  local blended_add_text = blend_color(add_fg, bg, 0.4)
-  local blended_del_text = blend_color(del_fg, bg, 0.4)
+  local blended_add_text = blend_color(add_fg, bg, 0.7)
+  local blended_del_text = blend_color(del_fg, bg, 0.7)
 
   vim.api.nvim_set_hl(0, 'DiffsAdd', { default = true, bg = blended_add })
   vim.api.nvim_set_hl(0, 'DiffsDelete', { default = true, bg = blended_del })
@@ -193,6 +193,15 @@ local function compute_highlight_groups()
   vim.api.nvim_set_hl(0, 'DiffsAddText', { default = true, bg = blended_add_text })
   vim.api.nvim_set_hl(0, 'DiffsDeleteText', { default = true, bg = blended_del_text })
 
+  dbg('highlight groups: Normal.bg=#%06x DiffAdd.bg=#%06x diffAdded.fg=#%06x', bg, add_bg, add_fg)
+  dbg(
+    'DiffsAdd.bg=#%06x DiffsAddText.bg=#%06x DiffsAddNr.fg=#%06x',
+    blended_add,
+    blended_add_text,
+    add_fg
+  )
+  dbg('DiffsDelete.bg=#%06x DiffsDeleteText.bg=#%06x', blended_del, blended_del_text)
+
   local diff_change = resolve_hl('DiffChange')
   local diff_text = resolve_hl('DiffText')