This bug is really annoying. I am in windows, using 10371. Both: Pharo and PharoCore, have the same problem.

To reproduce:

1) Go to squeak
2) alt + tab to switch to another windows
3) alt + tab again to come back to squeak

At that moment, we you get the focus in Squeak you will get TWO DNU. They are the same.

I attach SqueakDebug.log

Please consider this for 1.0 because it is unusable.

I created the ticket: http://code.google.com/p/pharo/issues/detail?id=945

Cheers,

Mariano