It's nearly impossible to position canvas 0° after you rotated it with finger gestures.
It would be very helpful to have locking at 0° and 90°. As an alternative every 45 degrees would be more useful.

How locking from user point of view can work:
1. When you pressed 2 fingers and started rotating - position locks at closest value (e.g. 0°) after you passed it by a few degrees (e.g. after 2°).
E.g. you start at -2° and rotate clockwise.
-2°: rotation
-1°: rotation
0°: no rotation - lock, no rotation happens even if you move your fingers in any direction
1° (or -1°): no rotation
2° (or -2°): no rotation
3° (or -3°): rotation is enabled again

2. To release lock (e.g. fine-tune between 0° and 2°) you first need to go beyond threshold and after return back.
E.g. you start at -2° and rotate clockwise first and counterclockwise after to fine tune the angle.
-2°: rotation
-1°: rotation
0°: no rotation - lock
1°: no rotation
2°: no rotation
3°: rotation is enabled again
2°: rotation (moving back in opposite direction to fine tune the angle)

STEPS TO REPRODUCE
1. Slightly rotate canvas with 2 fingers (e.g. by 5°)
2. Try positioning it back to 0°

OBSERVED RESULT
You will end up with 0.2° or -0.1° or any other angle but not straight.

EXPECTED RESULT
To be snapped on specific angles (0°, 45°, 90°, 135°)

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION