The editor grid will zfight with meshes that intersect the grid in the 3d viewport. Creates and unpleasant experience while working with the grid turned on.
Repro:
1. Launch Editor
2. create new map from template, blank, 3x3, or 5x5
3. observe the floor with grid visible, will be flickering
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-40627 in the post.