mw.ConfirmCloseWindow.js contains code that prevents the window from being closed while an upload is still in progress. It's just a function we call, once, so it's not UW-specific. This is too general a module to keep it locked up in UW.
This bug will take a patchset to core and a near-simultaneous patchset to UW. I would suggest writing both at once, for testing purposes, and making sure the UW one is ready to go first, then we can merge it immediately after the core patch is merged.
Alternatively we could add it to core, wait 'til it's merged, and only then try to remove the module from UW.
Either way, I want this code out of UW and into core so it's not gumming things up. I'm sure the core behaviour on the edit page is very similar, so it may be that there already is a module and it just needs some extra features, or that it could use this code instead and be just as effective.