Pressing F11 (in attempts to fullscreen) while playing in editor - new window causes the viewport in the editor itself to full screen as opposed to the new window that's created.
expectation is 1 of 2 behaviors:
-F11 with the new window focused doesn't do anything, but the 'fullscreen' command and/or Alt+Enter work
-F11 with the new window focused fullscreens the spawned window and not the viewport in the editor.
Not a regression (checked as far back as 4.5)
Result: Viewport in the editor is fullscreened behind the play window.
Expected Result: The new Editor window should be set to fullscreen
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-21499 in the post.