garbled rendering

qt sometimes corrupts the UI content as seen here:

• this is most easily observed with fonts (of any kind), but not exclusively
• some lines of the content are drawn a few pixels to the right or downwards
• the effect appears fairly randomly and goes away with the next repaint (cursor blinking is sufficient)
  • a somewhat reliable way to provoke the vertical variant:
    • open a window with lots of text - a file dialog is perfect
    • place it partially below another window
      • the position of the covering window is significant
    • start dragging it around with alt-left-mouse-button
    • the important thing here is the size/position overlay that kwin displays on top of the window, as this will cause frequent partial repaints
  • a somewhat reliable way to provoke the horizontal variant:
    • open konversation, make sure enough chat history is visible, possibly including a linkified url
    • place another window over much of konvi
    • move the mouse between the windows, so enter/leave events are generated
    • as before, the window positions are significant
• this affects all qt applications - you see a kde file dialog and konsole, also affected are for example konversation and qt creator
  • it does not appear to affect any non-qt apps
• this is happening with the intel driver; i never had this problem with the proprietary nvidia driver
• i'm under kwin_x11, compositing disabled

this actually got slightly better over the last months, where i did some kernel+mesa upgrades. i occasionally pulled the qt 5.9 branch, too.

i suspect this to be a driver bug, but given that only qt triggers it, there may be something to be done.