[Len Case]

The problem is, that undo states are generally window specific so closing one window (and thus focusing on another) puts you into an unpredicatable undo state--if the window you ended up on did have a text field focused, suddenly your undo wouldn't work to bring the window back whereas normally it could.

I have a phone where the scroll down a page button is also the accept call button (T-Mobile Sidekick)--sometimes while browsing a web page, I end up answering a phone call unexpectedly. Similarly, the switch to another call button shares functionality with the end call button. I've had someone on the line, get another phone call--go to switch after explaining to the first call--and when I go to switch, the other call has disappeared and I end up hanging up on the first person instead of putting them on hold.

This is the same reason the stop and reload buttons in OmniWeb are not the same button--you go to press on the stop and just before you get there, it turns into a reload. You don't want an unpredictable interface.