Stencils guides and rulers:
These are awesome - one thing that could improved when inverting a stencil, is to make the masked part of
the stencil extend a greater distance, maybe even full screen. right now if you invert an oval stencil, the masks
so thin that a large brush will paint over the outer edge of the stencil... I end up stacking a few around the edges
to prevent this.

Popup color picker (the one that shows up when pressing ctrl+alt+lmb):
I think this would work better if it were a toggle, so you didn't have to press shift to choose between hue and sat/value.
Could initiate the same way, just have a close button or have it disappear when cursor moves away from it.
Maybe some swatches along the outer edge with recent color picks, and the current color at the edges (no border) might
make comparing to canvas colors easier. (note - it would be great to have a tools version of the popup as well... see next)

Custom tool/brush palette:
the stencils have a favorites category - can this be done for tools?

Selection Modifier:
Use a hotkey to alternate between polygon select mode and freehand mode without switching tools in the settings panel. There is
another software that uses alt for this...

Anyway, great work! apologies if this is requested elsewhere....