On Maemo5/N900, Qt widgets containing GL GraphicsViews can be left undrawn in some cases. (I've only seen it happen in fullscreen mode, but that may be a coincidence.)

The attached sample application illustrates the issue. When run on an N900, it displays a GL QGraphicsView next to a QPushButton. Pull the N900 keylock switch quickly a few times until the app becomes totally black. (To reproduce, I pull the switch with about the same interval as how long it takes for the backlight to come on, slightly faster.)

When the issue has occurred, you'll only notice the backlight coming on/going off, nothing else. The GraphicsView that earlier was mostly white now is totally black, and the pushbutton isn't drawn either. Try clicking on the screen where the Test button was visible earlier - it becomes visible again when pressed.