app/internal/window: [macOS] avoid racy access to window width, height, scale

The macOS redraw callback is not invoked on the main thread, so its
access to window fields must be synchronized.

An alternative would be to schedule the asynchronous redraws on the main
thread, but I believe frame callbacks are performance-sensitive enough
to warrant the extra locking complexity.

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

app/internal/window/os_macos.go

@@ -48,9 +48,12 @@ func init() {
 }
 
 type window struct {
-	view          C.CFTypeRef
-	w             Callbacks
-	stage         system.Stage
+	view  C.CFTypeRef
+	w     Callbacks
+	stage system.Stage
+
+	// mu protect the following fields
+	mu            sync.Mutex
 	scale         float32
 	width, height float32
 }
@@ -204,8 +207,12 @@ func gio_onMouse(view C.CFTypeRef, cdir C.int, cbtns C.NSUInteger, x, y, dx, dy
 //export gio_onDraw
 func gio_onDraw(view C.CFTypeRef) {
 	w := mustView(view)
-	w.scale = float32(C.gio_getViewBackingScale(w.view))
-	w.width, w.height = float32(C.gio_viewWidth(w.view)), float32(C.gio_viewHeight(w.view))
+	scale := float32(C.gio_getViewBackingScale(w.view))
+	width, height := float32(C.gio_viewWidth(w.view)), float32(C.gio_viewHeight(w.view))
+	w.mu.Lock()
+	w.scale = scale
+	w.width, w.height = width, height
+	w.mu.Unlock()
 	w.draw(true)
 }
 
@@ -216,13 +223,15 @@ func gio_onFocus(view C.CFTypeRef, focus C.BOOL) {
 }
 
 func (w *window) draw(sync bool) {
-	wf, hf := w.width, w.height
+	w.mu.Lock()
+	wf, hf, scale := w.width, w.height, w.scale
+	w.mu.Unlock()
 	if wf == 0 || hf == 0 {
 		return
 	}
-	width := int(wf*w.scale + .5)
-	height := int(hf*w.scale + .5)
-	cfg := configFor(w.scale)
+	width := int(wf*scale + .5)
+	height := int(hf*scale + .5)
+	cfg := configFor(scale)
 	cfg.now = time.Now()
 	w.setStage(system.StageRunning)
 	w.w.Event(FrameEvent{