Le problème se répète. L'écran s'est mis à clignoter avant de rendre l'âme. Les logs disent:
févr. 03 11:25:05 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 49717, resource id: 14814282, major code: 3 (GetWindowAttributes), minor code: 0
févr. 03 11:25:05 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 49718, resource id: 14814282, major code: 14 (GetGeometry), minor code: 0
févr. 03 11:25:07 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 50374, resource id: 14814148, major code: 3 (GetWindowAttributes), minor code: 0
févr. 03 11:25:07 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 50375, resource id: 14814148, major code: 14 (GetGeometry), minor code: 0
févr. 03 11:25:16 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 51434, resource id: 14814427, major code: 3 (GetWindowAttributes), minor code: 0
févr. 03 11:25:16 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 51435, resource id: 14814427, major code: 14 (GetGeometry), minor code: 0
févr. 03 11:25:24 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 54341, resource id: 14814394, major code: 3 (GetWindowAttributes), minor code: 0
févr. 03 11:25:24 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 54342, resource id: 14814394, major code: 14 (GetGeometry), minor code: 0
févr. 03 11:25:32 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 56443, resource id: 14814688, major code: 3 (GetWindowAttributes), minor code: 0
févr. 03 11:25:32 localhost.localdomain kwin_x11[1367]: qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 56444, resource id: 14814688, major code: 14 (GetGeometry), minor code: 0
puis le PC s'est bloqué.