Hi everybody,
I’ve just registered to report the same “bug”, both in ROOT 5.30 and 5.32, with Qt4 under Ubuntu 11.04; I found this topic so I add my story here instead of opening a new one… basically, I use Qt to monitor an acquisition system, and a TQtWidget for displaying signal waveforms using histograms; the code that worked with ROOT 5.28 causes the histograms to overlap with newer versions - the same effect discussed in [QtRoot] Canvas update problem. Actually, there was only one histogram in the canvas, that I could move and manipulate, but every change I made was somehow “impressed” in the background.
The solution provided by artur worked for me, and now my “oscilloscope” is up and running… thanks! 
(By the way, I had to upgrade ROOT from 5.28 due to repeated “segmentation violation” related to TQtWidget… but this now belongs to the past!)