This is a regression as it does not occur in 4.14.3 at CL 3248269
Update chris.wood:
I think this might be a wobble caused by floating point errors in SMenuAnchor::Tick()
Result: The dropdown consistently jitters causing it to look blurry.
Repro note (chris.wood):
Problem wasn't with specific menus or circumstances, just when the precise layout meant a floating point error caused a one pixel jitter. It could happen to any menus but was common enough for me to easily repro. Didn't see it happening with right-click context menus. Happened with maybe 25-50% of combo/anchored menus.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-40293 in the post.