Hi, oldie but goodie, forgot about this earlier.
1. Have two layers each with something on it, but only one is visible.
2. Enter quickmask mode (q)
3. 'Select' something using standard brush tool.
4. Enable the disabled layer
5. Notice, that the screen will not update unless you zoom in-out.
6. If you don't do antyhing, and exit quickmask you can see screen refreshes on chunks where the brush hoovers over only.