If a Menu is long, or if the thing that opens it (e.g. a Select handle or a Lookup input) is close to the bottom of the screen, the menu will extend past the bottom of the screen. When this happens, some of the items are cut off and are not reachable. Sometimes the user can work around this by scrolling down so that the thing the menu is anchored to moves up, but that isn't possible if the anchor is near the bottom of the page, or near the bottom of the scrollable area it's in, or is in a dialog.
We should explore options for automatically resizing menus so that they don't extend past the bottom of the screen, similar to what OOUI does. Alternatively, we can explore whether menus should open upwards instead of downwards in certain cases, e.g. when a Select is near the bottom of the screen.
This has been reported in various forms, see T312121 (worked around in another way) and T341944 (combines this issue with another issue).