Section separator not displayed if section break follows submenu

[freakboy3742]

Describe the bug

If you have a menu that contains a submenu in a section, a section change before or after the submenu will not cause a section break. As a result, the separator bar will not be displayed.

Steps to reproduce

Generate a menu where
Reproduction case:

import toga


class TestApp(toga.App):
    def do_stuff(self, widget, **kwargs):
        pass

    def startup(self):
        cmd1 = toga.Command(
            self.do_stuff, text="Action 1", tooltip="Perform action 1", section=1
        )
        cmd2 = toga.Command(
            self.do_stuff, text="Action 2", tooltip="Perform action 2", section=2
        )

        sub_menu = toga.Group("Sub Menu", parent=toga.Group.COMMANDS, section=3)
        cmd3 = toga.Command(
            self.do_stuff,
            text="Action 3",
            tooltip="Perform action 3",
            group=sub_menu,
        )
        cmd4 = toga.Command(
            self.do_stuff,
            text="Action 4",
            tooltip="Perform action 4",
            group=sub_menu,
        )

        cmd5 = toga.Command(
            self.do_stuff, text="Action 5", tooltip="Perform action 5", section=4
        )
        cmd6 = toga.Command(
            self.do_stuff, text="Action 6", tooltip="Perform action 6", section=5
        )

        self.commands.add(cmd1, cmd2, cmd3, cmd4, cmd5, cmd6)
        self.main_window = toga.MainWindow()
        self.main_window.content = toga.Box()
        self.main_window.show()


def main():
    return TestApp("Test App", "org.beeware.test")


if __name__ == "__main__":
    main().main_loop()

This has 5 "top level" menus; each of which is in a different section; Section 3 is a submenu. There should be a separator between each

Expected behavior

There should be a section separator line between "Action 2" and "Sub Menu", and between "Sub Menu" and "Action 5", as those three items are all in different sections.

Screenshots

macOS:

GTK:

Winforms:

Interestingly, GTK does draw the separator for the item after the menu, due to a slight discrepancy in how the menu is constructed.

Environment

• Operating System: All desktop
• Python version: Any (tested on 3.10)
• Software versions:
  • Toga: 0.4.0

Logs

n/a

Additional context

The issue appears to lie in CommandSet.__iter__(). The logic for emitting GROUP_BREAK and SECTION_BREAK doesn't correctly account for cases where a group break is preceded or followed by a section break.

There is also an inconsistency in how menus are constructed on GTK.