Operating system: Linux-4.15.0-45-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce RTX 2080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 418.43
Broken: version: 2.80 (sub 58), branch: blender2.7, commit date: 2019-04-21 20:30, hash: rB797539e76ccf
Worked: version: 2.79
When using the "Quick Edit" option of the Texture Paint mode in 2.80, the screenshot created and loaded into the image editor is a literal viewport screenshot.
This includes the grid background and overlays.
Previously (in 2.79 or earlier), Quick Edit would modify the screenshot so that the background is turned to alpha. Since alpha is ignored by Quick Edit, this helped to avoid artifacts when painting over the texture.
Having the previous behavior back (no grid background) would be great already, but perhaps removing overlays completely from the Quick Edit screenshot would be even better (or at least those overlapping the object you're texture painting).