When you use stylesheets to style the QMainWindow background-color to have gradient the window flickers black when resizing. It looks like the window is first painted black and then filled with color.
With solid background color the resizing is smooth. With the solid background the result looks better, but it still flickers. It looks like the window is painted twice and that causes the flickering. The problem is more visible with complex UIs. The problems seems to be most distracting on Linux (RHEL6) with Qt 4.8.x, but it is noticeable also in Windows (Qt 4.8.x and 5.3.0).
The attached Gradient.zip example is simple to just demonstrate the problem. In real life examples the problem is more evident. Maximize or resize the window larger to see the problem.