Anecdotal, but I've found it happens when image scaling by a factor other than 2. Affects my setup of a 4K display at 2650x1440 effective resolution, or 1.5x scaling.
Exactly this. I run at 1920x1080 effective resolution on my 15" rMBP, and 1680x945 on my 13". I see the issue on both.
When I switch to the "default" scaling (1440x900 and 1280x800, respectively), it stops. Only the 15" has a discrete GPU, eliminating that as potentially causal.
