This has some usability issues, e.g. you drag a file accidentally to another box, then change your mind and drag it back while it is still being copied or moved

This has some usability issues, e.g. you drag a file accidentally to another box, then change your mind and drag it back while it is still being copied or moved in the original direction. This sort of thing. It looks like a simple and natural extension to what Synergy has, but in reality it's just a can of worms.