Feedback Case Number: <https://xojo.com/issue/65139>
I am very surprised that no one has noticed this bug yet. Sure, the scenario shown here of such a large menu is rather rare, but it does happen.
If you have a main menu item that contains so many entries that the menu editor scrolls vertically and you add a submenu to one of these entries, then the menu editor loses the ability to scroll vertically. This means that you can no longer select and edit entries outside the visible area.
Please compare both screenshots:
I have created a feedback case with a screen video and hope that Xojo fixes this real quick.