allow canvas rotate lock, or configurable canvas rotation keybinding
I've been hearing people complain that they accidentally pivot or rotate the canvas when they are trying to pan the image. This is because dragging the mouse button is for pan, but shift-middle-drag is for rotate.
Two possible mitigations occur to me: one is a "lock canvas rotation" in the View menu with an option to have it off by default; another is to use a less likely key combination, or at least allow it to be configurable, e.g. to shift-alt-middledrag.