Log the user-event queue's high-water mark

Now that the queue is unbounded, its depth is a useful signal for
understanding how the event loop behaves under load — and we expect it
to look very different across builds (e.g. master, which carries the
bounce-state-updates-to-ui-thread work, versus the v0.63.0 release this
fix ships in). Track the deepest the queue has ever been and log an Info
line whenever that record is broken, so the numbers show up in the log
for later reasoning. The mark is session-wide and doesn't reset when the
queue drains.

gocui has no logger of its own, so it exposes the new depth through a
handler (matching the existing SetFocusHandler / SetOpenHyperlinkFunc
pattern) that the gui registers to log via its own logger.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pkg/gocui/gui.go

@@ -603,6 +603,13 @@ func (g *Gui) SetRenderSearchStatusFunc(renderSearchStatusFunc func(*View, int,
 	g.renderSearchStatusFunc = renderSearchStatusFunc
 }
 
+// SetUpdateQueueHighWaterMarkHandler registers a diagnostic callback invoked
+// with the new depth whenever the queue of pending Update callbacks reaches a
+// new maximum. It may be called from any goroutine.
+func (g *Gui) SetUpdateQueueHighWaterMarkHandler(f func(depth int)) {
+	g.userEvents.setHighWaterMarkHandler(f)
+}
+
 // userEvent represents an event triggered by the user.
 type userEvent struct {
 	f    func(*Gui) error
@@ -639,6 +646,13 @@ type userEventQueue struct {
 	mutex    sync.Mutex
 	events   []userEvent
 	doorbell chan struct{}
+
+	// highWaterMark is the deepest the queue has ever been, and
+	// onHighWaterMark (if set) is called with the new depth each time that
+	// record is broken. Purely diagnostic: it lets us see how deep the queue
+	// gets in practice (see SetUpdateQueueHighWaterMarkHandler).
+	highWaterMark   int
+	onHighWaterMark func(int)
 }
 
 func newUserEventQueue() *userEventQueue {
@@ -649,14 +663,32 @@ func newUserEventQueue() *userEventQueue {
 func (q *userEventQueue) enqueue(ev userEvent) {
 	q.mutex.Lock()
 	q.events = append(q.events, ev)
+	newHighWaterMark := 0
+	if len(q.events) > q.highWaterMark {
+		q.highWaterMark = len(q.events)
+		newHighWaterMark = q.highWaterMark
+	}
+	onHighWaterMark := q.onHighWaterMark
 	q.mutex.Unlock()
 
+	// Report outside the lock: the handler does I/O (logging) and must not
+	// stall other producers or the draining loop.
+	if newHighWaterMark > 0 && onHighWaterMark != nil {
+		onHighWaterMark(newHighWaterMark)
+	}
+
 	select {
 	case q.doorbell <- struct{}{}:
 	default:
 	}
 }
 
+func (q *userEventQueue) setHighWaterMarkHandler(f func(int)) {
+	q.mutex.Lock()
+	q.onHighWaterMark = f
+	q.mutex.Unlock()
+}
+
 // dequeue pops the oldest event, reporting false when the queue is empty.
 func (q *userEventQueue) dequeue() (userEvent, bool) {
 	q.mutex.Lock()

pkg/gocui/user_event_queue_test.go

@@ -36,6 +36,37 @@ func TestUpdateIsUnboundedAndPreservesOrder(t *testing.T) {
 	assert.Equal(t, want, got)
 }
 
+// The high-water-mark handler fires only when the queue reaches a new maximum
+// depth, reporting that depth. It does not reset when the queue drains.
+func TestUpdateQueueHighWaterMark(t *testing.T) {
+	g := newTestGui(t)
+
+	var marks []int
+	g.SetUpdateQueueHighWaterMarkHandler(func(depth int) { marks = append(marks, depth) })
+
+	noop := func(*Gui) error { return nil }
+
+	// Three enqueues with no drain: new highs 1, 2, 3.
+	g.Update(noop)
+	g.Update(noop)
+	g.Update(noop)
+	_, err := g.processRemainingEvents()
+	assert.NoError(t, err)
+
+	// Two enqueues stay below the previous high of 3: no new marks.
+	g.Update(noop)
+	g.Update(noop)
+	_, err = g.processRemainingEvents()
+	assert.NoError(t, err)
+
+	// Four enqueues with no drain: only depth 4 beats the previous high.
+	for range 4 {
+		g.Update(noop)
+	}
+
+	assert.Equal(t, []int{1, 2, 3, 4}, marks)
+}
+
 // Concurrent producers must be able to enqueue safely (run under -race). Only
 // same-goroutine order is guaranteed, so we check that every event is delivered
 // exactly once and that each producer's own events stay in order.

pkg/gui/gui.go

@@ -414,6 +414,10 @@ func (gui *Gui) onNewRepo(startArgs appTypes.StartArgs, contextKey types.Context
 		return nil
 	})
 
+	gui.g.SetUpdateQueueHighWaterMarkHandler(func(depth int) {
+		gui.c.Log.Infof("User-event queue reached a new high-water mark: %d", depth)
+	})
+
 	gui.g.SetOnSelectSearchResultFunc(func(v *gocui.View, selectedLineIdx int) {
 		ctx, ok := gui.helpers.View.ContextForView(v.Name())
 		if ok {