Drop down menus clip on 27" Screen iMacs

Drop down menus clip on 27" Screen iMacs (See attached images.) Reported from a 27" iMac and tested on a 27" iMac here. Other models may be affected.

*Image with Blue circled areas are from the user. The other image was from Engine QA.

Regression: Does not occur in previous versions of the Engine.

1. Open UE4.15 on an 27" iMac 2015-2017
2. Open Editor so that it is full screen
3. Mouse over the drop down menus from the Menu bar at the top (File, Window, Help, etc.)
   RESULT: Drop down menus "clip" to show background elements SEE attached images.
   EXPECTED: Drop down menus to function so that they are entirely opaque over background elements.

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