After upgrading to KDE 4.4/4.5 I had a problem that my application which is using :0.0 and :0.1 (separate X screens) throws some X11 errors and not displaying anything when trying to open a QMenu or a QToolTip. The fist idea was that oxygen is doing something weird and after some digging I could fix it by not using a translucent background for QMenu/QToolTip. Digging deeper I soon came to QX11WindowSurface::flush(). I could confirm that this XCopyArea call is really the culprit. In the end I added a debug line just before the XCopyArea call:

QX11WindowSurface::flush()
{
...
    XSync(X11->display, false);
    qDebug("Before XCopyArea %p %p '%s' %d %d",d_ptr->device.handle(), widget->handle(), qPrintable(widget->objectName()), d_ptr->device.x11Info().screen(), widget->x11Info().screen());
    XCopyArea(X11->display, d_ptr->device.handle(), widget->handle(), gc,
              br.x() + offset.x(), br.y() + offset.y(), br.width(), br.height(), wbr.x(), wbr.y());
    XSync(X11->display, false);
    qDebug("After XSync");
...
}

The output was the following:
---------------8<---------------
Before XCopyArea 0x3c00042 0x3c0003b 'Menu for display 0' 1 0
X Error: BadMatch (invalid parameter attributes) 8
Major opcode: 62 (X_CopyArea)
Resource id: 0x3c00041
After XSync
---------------8<---------------

As you can see the source bitmap for the back buffer was created on the wrong screen. It is created in QX11WindowSurface::setGeometry() and there you can see that a call to QPixmap::x11SetDefaultScreen() is missing when there's a translucent background. After adding the appropriate call all works fine:

---------------8<---------------
QX11WindowSurface::flush 0x719f70, 0x7fff018c70c0
Before XCopyArea 0x3c0013c 0x3c00139 'Menu for display 0' 0 0
After XSync
---------------8<---------------

I've attached the testcase. You need two separate X screens :0.0 and :0.1 and the oxygen style (or a style which is using a translucent background for the QMenu/QToolTip) from KDE 4.4 or higher.

It would be nice to get this fix added to 4.6.x too if possible.

Thx,
Christian