Still resizes after releasing mouse outside window

[Fabioni]

Check out the sandbox: https://codesandbox.io/p/sandbox/react-resizable-panels-forked-r6zcm6

If you drag the resizer up until the first panel gets collapsed, and you release the mouse outside the iframe of codesandbox (or just outside your browser window) your code does not recognize the mouse release and still changes the panel size after the mouse hovers back over the iframe.

[bvaughn]

If you'd like to post a PR for this, I'll try to make the time to review it. Else I probably won't have the time to pick it up.