Thank you for your kind offer.
I have added some details:
System GMA 4500MHD (Lenovo T500 with Intel Core 2 Duo P8400. 3 GB DDR3 SDRAM, no additional graphics card), external monitor 1900x1200 32bit color, default text size.
Reproduced in IE11 on maps.google with
1) Scroll map at street level with street names showing
2) Before repainting completely finishes (appears to depend on drawing, not on http requests being answered), move the mouse over the bottom right symbols.
Specifically the symbol "?" at bottom right (opens popup "Welcome to the new Google Maps") and/or adjacent symbols.
Map areas will show glitches (about 30% of the times I tested, but once the glitches show they are very noticable).
3) Place the IE11 window over or under a window of a different process. The window of the other process with show glitches.
Cross testing with a Core i3 processor based notebook with a highly similar software environment showed no problems at all.
I tried replicating this issue on a similar system with GMA 4500 and Windows7 64bit with different browsers, But I did not get the same results.
I would suggest uninstalling the driver and running the windows default drivers just to see what behavior you get, but this does not seem to be a driver glitch.
Thank you, hansb_intel for your efforts, your clear answer and the solution.
I uninstalled the external monitor driver (not the notebook monitor driver), then the GM45 driver from the Windows device manager.
After a reboot the drivers re-installed automatically (drivers were not removed as files). A reboot was required.
The glitches can no longer be reproduced. I consider this solved.
Interestingly, previously the device manager had listed the device driver name without the "WDDM 1.1" notation.
The new driver name now is "Mobile Intel(R) 45 Express Chipset Family (Microsoft Corporation - WDDM 1.1)"
Cant say how I happy I am to have this solved after a long search.