Popup menus are displayed by default with their top at the mouse position, and go down until the end of the menu. But, sometimes, there is not enough room downwards and they open with their bottom at the mouse position and go up until the beginning of the menu. In these cases where the menu goes upwards instead of downwards, if the menu is tall enough, it can get hidden behind GNOME shell's top bar, making it impossible to read the top entries of the menu. However, even if the top entries are hidden, the good news is that the mouse cursor can still reach them and click them. But that's a bit awkward to have to click on an entry without being able to read its label...