ui/app: serialize event handling

The frame callback for macOS comes in from a different thread than
the UI thread. Serialize event handling to maintain the invariant
that only one event is processed at a time.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

ui/app/window.go

@@ -35,6 +35,8 @@ type Window struct {
 
 	events chan Event
 
+	eventLock sync.Mutex
+
 	mu           sync.Mutex
 	stage        Stage
 	size         image.Point
@@ -230,6 +232,8 @@ func (w *Window) contextDriver() interface{} {
 }
 
 func (w *Window) event(e Event) {
+	w.eventLock.Lock()
+	defer w.eventLock.Unlock()
 	w.mu.Lock()
 	needAck := false
 	needRedraw := false