This does not happen in a browser window, but I do see it in two places.

It happens when one is using a browser on a phone. One can click on the "Menu v' bar and then the items under it appear and immediately disappear, leaving the empty background of the menus visible.

It also happens, oddly, when I am in the theme configuration widget even when I am using a browser on my computer. I click on the 'Customize' button from the Themes page. It shows me a version of the main page. On that page, if I click on the menu bar, again the menus below appear but then immediately disappear.

To see it in action, use your phone and go to sanjoseuu.org and click on the Menu bar.