PostEmptyEvent from Canvas.SetDirty() as part of threading transition…

… plan

internal/driver/common/canvas.go

@@ -348,6 +348,13 @@ func (c *Canvas) CheckDirtyAndClear() bool {
 // SetDirty sets canvas dirty flag atomically.
 func (c *Canvas) SetDirty() {
 	c.dirty.Store(true)
+
+	// wake up main thread in case we were called from goroutine
+	// TODO: hide this behind the migration flag introduced in
+	// https://github.com/fyne-io/fyne/pull/5425
+	if drv, ok := fyne.CurrentApp().Driver().(interface{ PostEmptyEvent() }); ok {
+		drv.PostEmptyEvent()
+	}
 }
 
 // SetMenuTreeAndFocusMgr sets menu tree and focus manager.