Pressing f11 while playing in editor new window fullscreens the viewport in the editor

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)

1. Create a new project (No template or starter content needed)
2. Play in Editor in new window
3. Ensure you have the new window focused, then press F11

Result: Viewport in the editor is fullscreened behind the play window.
Expected Result: The new Editor window should be set to fullscreen

Fullscreen does not work in Play in Editor (New Window)