Viewports aren't updated (black/garbage) when resizing the editor windows

Found on Linux but Windows seems to have the same problems. When you are dragging/resizing the window, viewports stop updating and/or show garbage.

See https://answers.unrealengine.com/questions/216456/bug-viewport-particle-matinee-render-update-proble.html

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CL 2521711 is suspected to at least enable that problem, if not cause it.