Created attachment 371531 [details]
Switching from mouse to tablet on Gifcam window ("mouse" is drawn using mouse and "tablet" is attempted to be drawn using tablet

I linked some of the gifs in Bug 795677 but probably warrants this bug report.

Reproduction Steps
1. Open an external application that has an overlaid window. Gifcam is a good example since you can see the cursor underneath. I also tested using ShoeBox, a sprite packing tool.
2. Select any tool using tablet. Error is more prone to happen if mouse has a different tool selected. I usually had Rect Select on mouse and Paintbrush on tablet.
3. Draw on tablet making sure to go over the overlaid window at the same time.
4. The cursor will get confused when over the external window about whether or not you are using the tablet or the mouse. Sometimes you will see the paintbrush making rectangles that don't do anything. 
5. Do this enough time and Gimp will eventually crash.

Another attachment for reference: 
https://www.dropbox.com/s/y02q2o9hm3lck36/crash_context1.gif?dl=0
In this gif I am trying to reset the cursor's context by going over the menu/side, but since the cursor's context gets confused when using the tablet over the external window it doesn't help.