Toolbar at top of screen fails to respond

The initial symptom is that the buttons and links in the tool bar fail to respond.
However, if I view another tab and switch back, the section that should be covered by toolbar is not redrawn, and continues to show whatever was on that part of the other tab's screenful.

It's possible to fix this by force-reloading several times, but this has to be done each time the problem recurs.