We've just migrated a 3.5 application to Qooxdoo 4.0 and we've found a behavior change in selections that affects the way interaction occurs in that application.
That behavior change was necessary due to the touch support. Scrolling on a touch device starts with a touch on the item followed by a swipe. If the widgets change the selection on the first interaction e.g. mousedown / pointerdown the selection would
also change on a touch scroll which is an unexpected behavior. So this change was necessary and on intension. :)
Do you still having problems updating your code? If so, what are the issues?