Weird graphics all of a sudden

If anything that is drawn there stays there until overwritten.

Try setting a wallpaper.
I think the gnome way of doing it is launching system settings and clicking on appearance.

Cannot say for sure whether or not a program needs to actively tend to the root window, or if there's some X11 flag to clear the drawn pixels of off any window automatically as it is deleted.

I've always solved this issue with setting a wallpaper or a solid color.

I set my wallpaper again, and it seems to be back to normal. Thanks. :)

I worried it might be a failing or overloaded hardware component, but I don't notice any signs either physically or from within System Monitor.

Looks good now. :)

I am having a slightly differnet problem - on the task bar/menu, it should be that the start menu is first, then the icons, then the open programs, however it is the open programs first, then the icons (shortcuts to programs, then the start menu. How can I switch it around?