Resize grip next to an iframe: drag stalls and sticks

A splitter or resize grip whose mousemove/mouseup listeners sit on the host window breaks when the cursor crosses into an adjacent iframe. The iframe swallows the events, so the drag stalls, and a mouseup over the iframe is lost, so the pane keeps resizing on later moves with no button held. Synthetic test events don't catch this. Fix: while dragging, set pointer-events:none on the iframe (or use pointer events with setPointerCapture), end the drag on any move with buttons===0 or on window blur, and use box-sizing:border-box on the pane (otherwise each drag added 1px from the border).