Add a test for waiting on the UI thread after the loop has exited

Nothing dequeues user events once MainLoop has returned, so a worker
blocked in OnUIThreadAndWait is blocked for good. The assertion records
that; the next commit makes the wait give up instead.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pkg/gocui/ui_thread_test.go

@@ -0,0 +1,46 @@
+package gocui
+
+import (
+	"errors"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// errStillWaiting stands in for the result of a wait that hasn't produced one.
+var errStillWaiting = errors.New("still waiting")
+
+// resultOrTimeout reports what a wait returned, or errStillWaiting if it hasn't
+// returned by the time we give up on it.
+func resultOrTimeout(result chan error) error {
+	select {
+	case err := <-result:
+		return err
+	case <-time.After(time.Second):
+		return errStillWaiting
+	}
+}
+
+// A worker waiting for the UI thread must not be left parked there once the
+// main loop has stopped: nothing will ever run its callback, and the shutdown
+// that follows blocks until such workers have finished (see
+// tasks.ViewBufferManager.Close).
+func TestOnUIThreadAndWaitGivesUpWhenTheLoopExits(t *testing.T) {
+	g := newTestGui(t)
+
+	// Closing this is what MainLoop returning does. From here on nothing
+	// dequeues user events, so the callback below is never going to run.
+	close(g.loopExited)
+
+	result := make(chan error, 1)
+	go func() {
+		result <- g.OnUIThreadAndWait(func() {})
+	}()
+
+	err := resultOrTimeout(result)
+	/* EXPECTED:
+	assert.ErrorIs(t, err, ErrLoopExited)
+	ACTUAL: */
+	assert.ErrorIs(t, err, errStillWaiting)
+}