Dropdown selection box jitters when mouse is moved over top of it on Mac

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()

1. Create a new BP FirstPerson project on Mac
2. Resize window so that it does NOT fill the entire screen
3. Select a cube in the viewport
4. In the details panel click on the dropdown for Static Mesh

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.