I'm using two kinds of applications as an example: brave browser and dolphin (both having a hamburger-menu on the far right side of the application-window with additional submenu entries)

Setup: Place the right side of the applications exactly to the right side (edge) of the screen/monitor, so that there's no space left to the right.
Open the hamburger menu.

The popup-menu is rendered (see screenshots) so that the whole menu is visible, which means it is offset to the left to show all entries.
Position the mouse on a menu-item which has a submenu.
This submenu is always positioned to the rightmost monitor-edge.

- If this submenu has entries which are smaller than the parent-popup-menu, we can access the submenu entries and the parent menu entries with the mouse.
- if the submenu has entries similar in length as the parent, the submenu is opened similarly to an 'overlay' on the parent with nearly invisible borders. Problem: you cannot access any (downwards) parent-menu entries any more as they are covered by the submenu
- same for submenus larger then the parent

Compare this to the menu/submenu handling of brave:
Here, the submenu is rendered to the left side of the parent menu, if no space to the right side is available.

Long story short: make submenus appear to the opposite side of the parent menu, if the space available to the right to render the submenu is not sufficient.