GtkMenuButton widget focus behavior.
When user clicks on some button the focus is moved to the popover. However, when user clicks back on working area (for example on the page of the document in the Evince pdf viewer) the focus gets back to the previously focused widget, which in case of the Evince is the top bar. A more precise example is described in one of the Evince issues: [BUG] Move around a page shortcuts stop working after clicking on the file options button.
I am not sure if you consider this behavior as a bug, but in my opinion it is not what is expected by the user. For example I expect the focus to be on the element I have just clicked. @nbenitez