I just added some new features:
In keeping with ctrl holding aspect ratio constant, now dragging the top/bottom and left/right handles while holding ctrl allows for unidirectional adjustment of the crop location, limiting motion to vertically and horizontally, respectively.
For both these dragging and for the omnidirectional drag, I also added snapping that turns on via shift: if the crop is within 5% of the original image width or height of the center, then it’ll snap to the center. For omnidirectional snapping, each direction snaps to center independently.
I think this is it for actual user interaction stuff to program… I may also need to figure out how to split up the Crop button itself for accepting/rejecting the crop, but that’s for another day.
And then I have to do the backend.