Comment # 11 on bug 1176229 from Dirk S.

> Do you have Mesa-dri-nouveau installed?
No, it is locked because it doesn't support my graphics card:
VGA compatible controller: NVIDIA Corporation NV5 [Vanta / Vanta LT] (rev 15)

kwin_x11 is just running fine. If I kill it and restart it in a console window,
I'll get the follwing messages:
$ kwin_x11
qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 180, resource
id: 62914567, major code: 20 (GetProperty), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 181, resource
id: 62914567, major code: 20 (GetProperty), minor code: 0
kwin_core: Compositing is not possible
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1359, resource
id: 62914583, major code: 72 (PutImage), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1361, resource
id: 62914584, major code: 72 (PutImage), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1363, resource
id: 62914585, major code: 72 (PutImage), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1368, resource
id: 62914585, major code: 72 (PutImage), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1369, resource
id: 62914585, major code: 72 (PutImage), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1370, resource
id: 62914585, major code: 72 (PutImage), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1371, resource
id: 62914585, major code: 72 (PutImage), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1372, resource
id: 62914585, major code: 72 (PutImage), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 16 (BadLength), sequence: 1373, resource
id: 62914585, major code: 72 (PutImage), minor code: 0