Drag one of a rounded rectangle's or an image frame's corner knobs — the canvas owns where they are and what a drag
does (corner_knob_at / drag_corner_knob); this follows the mouse until it is released.
All four corners follow by default. Option or Cmd, read on every move, rounds only the corner
dragged — so a modifier pressed or let go mid-drag takes effect at once, as in Illustrator.
One undo step, opened on the first move so a click on a knob records nothing.
Drag one of a rounded rectangle's or an image frame's corner knobs — the canvas owns where they are and what a drag does (corner_knob_at / drag_corner_knob); this follows the mouse until it is released.
All four corners follow by default. Option or Cmd, read on every move, rounds only the corner dragged — so a modifier pressed or let go mid-drag takes effect at once, as in Illustrator. One undo step, opened on the first move so a click on a knob records nothing.