Operating system: Windows-7-6.1.7601-SP1 64 Bits
Graphics card: GeForce GTX 660 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 430.86
Broken: version: 2.80 (sub 75), branch: master, commit date: 2019-07-25 08:02, hash: rB4f363283eb2c
Broken: version: 2.79 (sub 7), branch: blender2.7, commit date: 2019-06-27 10:41, hash: e045fe53f1b0, type: Release
Short description of error
Mask brush in 2D paint editor just painting white, but not masking at all.
May be I am wrong, but it not documented at all, and there is question on BlenderArtists since 2015, without answer.
So it seems nobody know what is it for, how it work, but it is inside 2.80 now.
Exact steps for others to reproduce the error
Try to use it by yourself