refactor(renderer): extract extmark_clear_range and avoid redundant clears

Extract extmark_clear_range to compute the range of extmarks to clear and return clear_start/clear_end.
Change apply_extmarks to accept a skip_clear flag so callers can control when clearing happens.
Clear extmarks once before writing lines in upsert_message_now and upsert_part_now, and during bulk flush.
Add unit tests to assert extmarks are cleared before replay and that clear_extmarks is called before set_lines.

Author: sudo-tee

lua/opencode/ui/renderer/buffer.lua

@@ -129,7 +129,7 @@ local function highlight_written_lines(start_line, lines)
   output_window.highlight_changed_lines(start_line, start_line + #lines - 1)
 end
 
-local function apply_extmarks(previous_formatted, formatted_data, line_start, old_line_end, new_line_end)
+local function extmark_clear_range(previous_formatted, formatted_data, line_start, old_line_end, new_line_end)
   local prefix_len = math.min(
     unchanged_prefix_len(previous_formatted, formatted_data),
     unchanged_extmark_prefix_len(previous_formatted, formatted_data)
@@ -162,13 +162,21 @@ local function apply_extmarks(previous_formatted, formatted_data, line_start, ol
   if next_min_extmark ~= nil then
     clear_start = math.min(clear_start, line_start + next_min_extmark)
   end
+
   clear_start = math.max(0, clear_start)
   local clear_end = math.max(
     max_extmark_line(previous_formatted, old_line_end),
     max_extmark_line(formatted_data, new_line_end)
   ) + 1
 
-  output_window.clear_extmarks(clear_start, clear_end)
+  return clear_start, clear_end
+end
+
+local function apply_extmarks(previous_formatted, formatted_data, line_start, old_line_end, new_line_end, skip_clear)
+  local clear_start, clear_end = extmark_clear_range(previous_formatted, formatted_data, line_start, old_line_end, new_line_end)
+  if not skip_clear then
+    output_window.clear_extmarks(clear_start, clear_end)
+  end
 
   local extmark_start_line = math.max(0, clear_start - line_start)
   local extmarks = slice_extmarks(formatted_data.extmarks, extmark_start_line)
@@ -341,12 +349,20 @@ function M.upsert_message_now(message_id, formatted_data, previous_formatted)
     local prefix_len = unchanged_prefix_len(previous_formatted, formatted_data)
     local write_start = cached.line_start + prefix_len
     local lines_to_write = slice_lines(formatted_data.lines, prefix_len + 1)
-
+    local clear_start, clear_end = extmark_clear_range(
+      previous_formatted,
+      formatted_data,
+      cached.line_start,
+      old_line_end,
+      cached.line_start + #formatted_data.lines - 1
+    )
+
+    output_window.clear_extmarks(clear_start, clear_end)
     output_window.set_lines(lines_to_write, write_start, cached.line_end + 1)
     highlight_written_lines(write_start, lines_to_write)
 
     local new_line_end = cached.line_start + #formatted_data.lines - 1
-    apply_extmarks(previous_formatted, formatted_data, cached.line_start, old_line_end, new_line_end)
+    apply_extmarks(previous_formatted, formatted_data, cached.line_start, old_line_end, new_line_end, true)
     ctx.render_state:set_message(cached.message, cached.line_start, new_line_end)
 
     local delta = new_line_end - old_line_end
@@ -399,7 +415,15 @@ function M.upsert_part_now(part_id, message_id, formatted_data, previous_formatt
     local prefix_len = unchanged_prefix_len(previous_formatted, formatted_data)
     local write_start = cached.line_start + prefix_len
     local lines_to_write = slice_lines(formatted_data.lines, prefix_len + 1)
-
+    local clear_start, clear_end = extmark_clear_range(
+      previous_formatted,
+      formatted_data,
+      cached.line_start,
+      old_line_end,
+      cached.line_start + #formatted_data.lines - 1
+    )
+
+    output_window.clear_extmarks(clear_start, clear_end)
     output_window.set_lines(lines_to_write, write_start, cached.line_end + 1)
     highlight_written_lines(write_start, lines_to_write)
 
@@ -409,7 +433,7 @@ function M.upsert_part_now(part_id, message_id, formatted_data, previous_formatt
     if new_line_end ~= cached.line_end then
       ctx.render_state:update_part_lines(part_id, cached.line_start, new_line_end)
     end
-    apply_extmarks(previous_formatted, formatted_data, cached.line_start, old_line_end, new_line_end)
+    apply_extmarks(previous_formatted, formatted_data, cached.line_start, old_line_end, new_line_end, true)
     set_part_extmark_state(part_id, formatted_data)
     return true
   end

lua/opencode/ui/renderer/flush.lua

@@ -436,6 +436,8 @@ function M.end_bulk_mode()
       output_window.set_lines(lines, 0, -1)
     end)
 
+    output_window.clear_extmarks()
+
     if next(ctx.bulk_extmarks_by_line) then
       output_window.set_extmarks(ctx.bulk_extmarks_by_line, 0)
     end

tests/unit/output_window_spec.lua

@@ -251,3 +251,37 @@ describe('renderer flush cleanup', function()
     assert.stub(set_extmarks_stub).was_not_called()
   end)
 end)
+
+describe('renderer bulk flush extmarks', function()
+  local buf
+
+  before_each(function()
+    buf = vim.api.nvim_create_buf(false, true)
+    state.ui.set_windows({ output_buf = buf })
+    vim.api.nvim_buf_set_lines(buf, 0, -1, false, { 'old header', 'old body', '' })
+    vim.api.nvim_buf_set_extmark(buf, output_window.namespace, 2, 0, {
+      virt_text = { { 'OLD', 'Normal' } },
+      virt_text_pos = 'overlay',
+    })
+  end)
+
+  after_each(function()
+    local ctx = require('opencode.ui.renderer.ctx')
+    ctx:reset()
+    state.ui.set_windows(nil)
+    pcall(vim.api.nvim_buf_delete, buf, { force = true })
+  end)
+
+  it('clears stale extmarks before replaying bulk extmarks', function()
+    local ctx = require('opencode.ui.renderer.ctx')
+
+    flush.begin_bulk_mode()
+    ctx.bulk_buffer_lines = { 'new header' }
+
+    flush.end_bulk_mode()
+
+    local marks = vim.api.nvim_buf_get_extmarks(buf, output_window.namespace, 0, -1, { details = true })
+    assert.equals(0, #marks)
+    assert.same({ 'new header', '' }, vim.api.nvim_buf_get_lines(buf, 0, -1, false))
+  end)
+end)

tests/unit/renderer_buffer_spec.lua

@@ -3,16 +3,40 @@ local ctx = require('opencode.ui.renderer.ctx')
 local output_window = require('opencode.ui.output_window')
 local stub = require('luassert.stub')
 
+local function assert_called_before(call_order, first_name, second_name)
+  local first_idx
+  local second_idx
+
+  for idx, name in ipairs(call_order) do
+    if name == first_name and not first_idx then
+      first_idx = idx
+    end
+    if name == second_name and not second_idx then
+      second_idx = idx
+    end
+  end
+
+  assert.is_truthy(first_idx, 'expected ' .. first_name .. ' to be called')
+  assert.is_truthy(second_idx, 'expected ' .. second_name .. ' to be called')
+  assert.is_true(first_idx < second_idx)
+end
+
 describe('renderer.buffer extmarks', function()
   local set_lines_stub
   local clear_extmarks_stub
   local set_extmarks_stub
   local highlight_changed_lines_stub
+  local call_order
 
   before_each(function()
     ctx:reset()
-    set_lines_stub = stub(output_window, 'set_lines')
-    clear_extmarks_stub = stub(output_window, 'clear_extmarks')
+    call_order = {}
+    set_lines_stub = stub(output_window, 'set_lines').invokes(function()
+      call_order[#call_order + 1] = 'set_lines'
+    end)
+    clear_extmarks_stub = stub(output_window, 'clear_extmarks').invokes(function()
+      call_order[#call_order + 1] = 'clear_extmarks'
+    end)
     set_extmarks_stub = stub(output_window, 'set_extmarks')
     highlight_changed_lines_stub = stub(output_window, 'highlight_changed_lines')
   end)
@@ -48,6 +72,7 @@ describe('renderer.buffer extmarks', function()
         { line_hl_group = 'OpencodeReasoningText' },
       },
     }, 11)
+    assert_called_before(call_order, 'clear_extmarks', 'set_lines')
   end)
 
   it('reapplies extmarks at the correct line after unchanged leading lines', function()
@@ -77,5 +102,27 @@ describe('renderer.buffer extmarks', function()
         { line_hl_group = 'OpencodeDialogOptionHover' },
       },
     }, 24)
+    assert_called_before(call_order, 'clear_extmarks', 'set_lines')
+  end)
+
+  it('clears extmarks before rewriting a message', function()
+    ctx.render_state:set_message({ info = { id = 'msg_1' } }, 30, 31)
+
+    buffer.upsert_message_now('msg_1', {
+      lines = { 'alpha', '' },
+      extmarks = {},
+      actions = {},
+    }, {
+      lines = { 'alpha', 'beta' },
+      extmarks = {
+        [1] = {
+          { virt_text = { { 'OLD', 'Normal' } }, virt_text_pos = 'overlay' },
+        },
+      },
+      actions = {},
+    })
+
+    assert.stub(clear_extmarks_stub).was_called()
+    assert_called_before(call_order, 'clear_extmarks', 'set_lines')
   end)
 end)