fix(renderer): parts occasionally not rendering

This was a subtle one and it only shows up when we're not collapsing
events so it wasn't caught by the unit tests. I've since modified the
unit tests to also run the replays without collapsing to catch bugs like
this in the future.

If the first version of a part didn't result in something being
rendered, future updates to that part would also not render anything.
This was particularly likely to happen with tools as the first version
of the part is often `step-start`. Because we didn't think this was a
new part, we called `_replace_part_in_buffer` but that exits quickly
because the part hadn't been rendered anywhere.

While we could try to call `_insert_part_to_buffer` in
`_replace_part_in_buffer` if the part hadn't been rendered, that would
miss some subtle cases around error handling so the correct fix is
consider the part new in `on_part_updated` if we haven't rendered it
before.

Author: cameronr

lua/opencode/ui/render_state.lua

@@ -243,16 +243,18 @@ end
 
 ---Update part data reference
 ---@param part_ref OpencodeMessagePart New part reference (must include id)
+---@return RenderedPart? part The rendered part
 function RenderState:update_part_data(part_ref)
   if not part_ref or not part_ref.id then
     return
   end
-  local part_data = self._parts[part_ref.id]
-  if not part_data then
+  local rendered_part = self._parts[part_ref.id]
+  if not rendered_part then
     return
   end
 
-  part_data.part = part_ref
+  rendered_part.part = part_ref
+  return rendered_part
 end
 
 ---Helper to update action line numbers

lua/opencode/ui/renderer.lua

@@ -280,6 +280,7 @@ end
 function M._replace_part_in_buffer(part_id, formatted_data)
   local cached = M._render_state:get_part(part_id)
   if not cached or not cached.line_start or not cached.line_end then
+    -- return M._insert_part_to_buffer(part_id, formatted_data)
     return false
   end
 
@@ -536,7 +537,13 @@ function M.on_part_updated(properties, revert_index)
   if is_new_part then
     M._render_state:set_part(part)
   else
-    M._render_state:update_part_data(part)
+    local rendered_part = M._render_state:update_part_data(part)
+    -- NOTE: This isn't the first time we've seen the part but we haven't rendered it previously
+    -- so try and render it this time by setting is_new_part = true (otherwise we'd call
+    -- _replace_message_in_buffer and it wouldn't do anything because the part hasn't been rendered)
+    if not rendered_part or (not rendered_part.line_start and not rendered_part.line_end) then
+      is_new_part = true
+    end
   end
 
   local formatted = formatter.format_part(part, message, is_last_part)

tests/unit/renderer_spec.lua

@@ -3,6 +3,7 @@ local ui = require('opencode.ui.ui')
 local helpers = require('tests.helpers')
 local output_window = require('opencode.ui.output_window')
 local assert = require('luassert')
+local config = require('opencode.config')
 
 local function assert_output_matches(expected, actual, name)
   local normalized_extmarks = helpers.normalize_namespace_ids(actual.extmarks)
@@ -120,19 +121,29 @@ describe('renderer', function()
       local expected_path = 'tests/data/' .. name .. '.expected.json'
 
       if vim.fn.filereadable(expected_path) == 1 then
-        it('replays ' .. name .. ' correctly (event-by-event)', function()
-          local events = helpers.load_test_data(filepath)
-          state.active_session = helpers.get_session_from_events(events)
-          local expected = helpers.load_test_data(expected_path)
-
-          helpers.replay_events(events)
-          vim.wait(1000, function()
-            return vim.tbl_isempty(state.event_manager.throttling_emitter.queue)
-          end)
-
-          local actual = helpers.capture_output(state.windows and state.windows.output_buf, output_window.namespace)
-          assert_output_matches(expected, actual, name)
-        end)
+        for i = 1, 2 do
+          config.ui.output.rendering.event_collapsing = i == 1 and true or false
+          it(
+            'replays '
+              .. name
+              .. ' correctly (event-by-event, '
+              .. (config.ui.output.rendering.event_collapsing and 'collapsing' or 'no collapsing')
+              .. ')',
+            function()
+              local events = helpers.load_test_data(filepath)
+              state.active_session = helpers.get_session_from_events(events)
+              local expected = helpers.load_test_data(expected_path)
+
+              helpers.replay_events(events)
+              vim.wait(1000, function()
+                return vim.tbl_isempty(state.event_manager.throttling_emitter.queue)
+              end)
+
+              local actual = helpers.capture_output(state.windows and state.windows.output_buf, output_window.namespace)
+              assert_output_matches(expected, actual, name)
+            end
+          )
+        end
 
         if not vim.tbl_contains(skip_full_session, name) then
           it('replays ' .. name .. ' correctly (session)', function()