There is a frameless window (Qt::FramelessWindowHint), which is hidden and shown when needed. When a QGraphicsDropShadowEffect is set to the window, the widgets in it are not updated when the window is hidden and shown again. Both Qt::FramelessWindowHint and QGraphicsDropShadowEffect are needed to reproduce the problem.
If you hover a mouse over the non-updated widgets, a repaint is triggered. Also when show() is called while the window is shown, the update happens as expected.
Reproducible on Windows 7, but not on Linux (Ubuntu 14.04).
The attached example can be used to reproduce. There is a key event counter, which is shown in 3 widgets in a window when space key is pressed. Now, press some keys in the main window and then 'space', the widgets in the popup window show correct number. Repeat random key presses and 'space', and now the widgets are not updated. Hover over the widgets or press 'space' again, and the counters are updated.