setPointerCapture

Keep sending this pointer's events to element until it is released, even once the pointer leaves the element's box.

Without it, dragging a resize handle past the edge of the canvas silently stops updating: the events start going to whatever is under the cursor. Every drag the editor starts takes capture and releases it on pointer up.

Never throws. Capture legitimately fails when the pointer has already been lifted — a fast drag out of the window — and there is nothing useful to do about that but carry on.