When editing photos and using a Photos plugin, such as Pixelmator, there is about a 70% chance that the app will freeze when returning, requiring a user to force quit Photos. When this happens, the “reloading/refreshing” rectangular dialog that appears is blank (no text). There is also no CPU activity (according to Activity Monitor) and it just appears as though the dialog is stuck, which prevents any clicking within the app. I can reproduce it repeatedly and it happens on a completely fresh install of Mac OS (fully patched).