[bug]: Rectangle "mousedown" persists when cursor moved beyond canvas about invokeai HOT 2 OPEN

This is partially resolved by #6311. The rect snaps to the edges of the canvas.

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I agree it should work like this but it's rather finicky. Mouse events are handled by the canvas itself and moving the mouse outside the canvas doesn't trigger them, so we don't get mouseup or mousemove events when the cursor leaves. We do get a mouseleave with some coordinates.

The way to handle this is to snap to the edges of the canvas when using the rectangle tool, and not hide the rect if the mouse leaves the stage. This was a bit much to implement at the early stage when I added this tool but now that the dust has settled it shouldn't be too hard to do.

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