Hello,
I had the flickering drop-down menus problem (UE 4.25.4 - 4.26.0) running on Windows 7, spreading to a whole screen sometimes. The solution which works for me: Windows Control Panel > All Control Panel Items > System > Advanced system settings > Advanced > Performance > UNCHECK “Enable desktop composition” et voila!
Then you can even enable the windows animation in UE4 Editor Preferences and it will work.
I hope that it will help you too!