Many users find convenient to flip the tablet stylus pen and use the eraser to erase portion of a drawing. As of the latest build of krita, however (2.7.9.5 Win64), using the eraser causes the brush size selection to get locked/disabled (the setting in the toolbar becomes gray and unselectable) under certain conditions.

One of these is that as soon as the spacebar canvas panning is used (default key: spacebar), flipping the pen and using the eraser causes the brush size selection to get disabled as soon as the tip end of the pen is used again.

So far I haven't been able to easily reproduce this bug without using the spacebar canvas panning function first.

Reproducible: Always

Steps to Reproduce:
1. Create a new image 
2. Paint something
3. Press spacebar to pan the canvas at least once
4. Lift the pen, give proximity to the eraser end of the stylus
5. Hover the pen/erase something
6. Lift the pen, give proximity to the stylus' tip end
Actual Results:  
The brush size becomes locked to the brush preset's default setting, no matter the preset selected.

Expected Results:  
Brush size should never get locked.

Restarting Krita or reloading the current image gets around this problem.