Menus cast a shadow over the widget which opened them

GTK 4 and Qt 6 ameliorate this by having a relatively small drop shadow on menus, and by shifting that drop shadow downwards. GTK shifts the whole shadow down by about half of its radius, while Qt moves the shadow's top edge down by its entire radius.

In both cases, the same vertical shadow offset is applied to submenus. This means that submenus are allowed to shadow the right edge of their parent menu.

No file or test is named. Start by locating the menu rendering and shadow handling in the Rust code, then compare the current placement with the GTK 4 and Qt 6 examples; done means menus no longer shadow the widget that opened them while submenus retain the described right-edge behavior.

Written by the indexing model from the issue text.