Escape the page leaves unhandled reaches the window as cancelOperation:, and NSWindow leaves full screen on it, so pressing Escape once more after closing a popup dropped the IDE out of full screen. The IDE's windows now answer cancelOperation: with nothing; View › Appearance › Toggle Full Screen (⌃⌘F) leaves full screen. A page-level preventDefault was not used because several windows close on an otherwise unhandled Escape.
Fixes#3104