There are a lot of rendering bugs with QT_ONSCREEN_PAINT=force. For instance, if you play around (move, resize, etc.) with the windows created from the code below, some pieces of windows will sometimes vanish as if there were a square hole in them.

#include <QtGui>

int main(int argc, char *argv[])
{
	QApplication app(argc, argv);
	QWidget widget1;
	QLabel *label1 = new QLabel("label1", &widget1);
	widget1.showFullScreen();
	QWidget widget2;
	QLabel *label2 = new QLabel("label2", &widget2);
	widget2.show();

	return app.exec();
}

This issue is even worse with QT_ONSCREEN_PAINT=1.

There are also small issues with widget1.show() instead of widget1.showFullScreen(), like pieces of windows not replaced by the background desktop when the window is moved away, or pieces of a window not repainted correctly when the other window is moved above.

Note 1: There is no such issue without QT_ONSCREEN_PAINT.

Note 2: You may have to insist a little bit and to pay attention to details to reproduce the issue.