I have started to experience this in AppKit (Cocoa for macOS) app development on BigSur. Here's a typical scenario: User launches the app Clicks the tiny green button in the upper-left of main window title bar to make app fullscreen. Presses cmd + ',' or (manages to reach the Preferences menu item while in full-screen mode) Preferences window pops up as a modal window (via: contentViewController..presentAsModalWindow(preferencesViewController) Prefs view appears fullscreen with an animation (sliding in from right to left) User closes the prefs window pressing OK or Cancel (both of which end up firing self.dismiss() where self is the preferencesViewController) A huge black blank screen in complete nothingness - the only way to gid rid of it is quitting the application with cmd + Q Number 7 only occurs on BigSur and not in previous versions of macOS (tested as with as low as 10.13)