Tooltips shadow stuck on desktop

First a remark : Instead of restarting Windows, one can just restart Explorer. In the Task Manager, kill explorer.exe, then restart it using the menu entry File / New task.

A sure way to solve the problem is to simply disable shadows on tooltips :

1. Start Control Panel -> Ease of Access Center, or just press Windows logo key+U.
2. Under "Explore all settings", click "Make the computer easier to see".
3. Under "Make things on the screen easier to see", click "Fine tune display effects".
4. Click the "Effects" button.
5. In Effects, uncheck "Show shadows under menus".
6. OK all the way out.

instead of change resolution, can you try change color depth, and see if it works? Once the shadow is gone, you can just say "no" to the new color depth.

I believe I solve this each time by getting it to appear again, from the same program: It causes the previous one to disappear.

If I remember correctly, there was an options in the windows performance settings to disable tooltip shadows.