Ah well, I should have tested it myself earlier, too. I was really hoping it would work! I’ll try to gather details and submit a bug report here:
but it could really take quite a long time to get the right eyeballs on it. GTK is a huge project with many many bugs already in the queue. Fortunately there is a dev that just recently submitted a patch for MacOS and I believe he is working on Windows issues too. I’m going to try to help him out if I can:
Sort of related. . . I spent the afternoon making this patch for MyPaint:
The patch is for pressure but could easily be extended for rotation, etc. Basically my thinking is if you are using a mouse or an unsupported device it is no fun to just have opacity slider and keyboard control since so many other settings might be controlled by pressure (smudge, etc). With this patch I can use a mouse and control the pressure via ctrl+s and ctrl+a and it’s really not as terrible as it sounds. Ok it’s no replacement for a stylus but it makes using a mouse or totally unsupported tablet (MacOS) much better.
So, I wonder how useful it would be to adjust rotation via some keyboard shortcuts. . . it would certainly be slower than just twisting the pen. . . but maybe have it jump in 22.5 degree increments so you can quickly switch from broad to thin in 4 taps? (4x22.5=90). Maybe worth a shot?