First things first: Steam version, using Wacom Cintiq 24HD, windows 7
So, I will be drawing and randomly the brush tool tit vanishes, and I can't paint any more. I can move my cursor, use the menus, change settings, but the second I apply to the canvas nothing happens. Is fixed by restarting. Nothing else seems to get me back to being able to paint again.
-UPDATE-
So I found the cause, but it's still infuriating. If you accidentally press the side button (or on mouse, the other mouse button) while using the main control button, it loses all input and you can't get it back without a lot of fiddling and mashing of both buttons. I have to disable rightclick on my tablet pen, but it's still a really silly problem that should be fixed.
Getting back the controls is very hit and miss though once you accidentally trigger this bug.