I have a bit of an older version, so this may have already been corrected. If so, please ignore.
Compositing two images and using the mask Paint Tool.
There are two buttons available related to the accumulation of a stroke. One allows the user to hold the tool in the same spot and by repeat clicking, add further density to the mask while the other has no effect to the mask density (at least that is how I understand it).
"Brush Strokes Don't Accumulate" seems to be the default, but when using this button the repeated clicks do accumulate and cause greater density in the mask.
If, however, one clicks on the Brush Strokes Accumulate button (which works as intended), then goes back to the first button, the button then seems to act as intended.
Mask Paint Tool
Moderator: jsachs