Inconsistent state of event loop across platforms during user startup

[rmartin16]

Describe the bug

The status of the event loop at the time user startup runs is not consistent across platforms. On Linux, iOS, Android, Web, and Textual, the event loop is running when the user's startup code runs....but it is not running on macOS and Windows. Therefore, users cannot use operations that require a running event loop such as calling asyncio.create_task().

Steps to reproduce

Call asyncio.create_task() from toga.App.startup().

Expected behavior

Ideally, the status of the event loop would be consistent across platforms. When it's not, users may experience confusing errors on one platform while their code works on another.

That said, I wouldn't consider this critical. But given this, we should be careful not to recommend operations in startup that require a running event loop to users or in docs. Additionally, now that toga.App.on_running() exists, users can simply do these types of things there instead of in startup().

Screenshots

No response

Environment

• Operating System: all
• Python version: all
• Software versions:
  • Briefcase: 0.3.20.dev207
  • Toga: 0.4.7.dev59+gf379c61e7

Logs

No response

Additional context

Feel free to close if these differences aren't that important or if they are simply inherent to the platform.

[freakboy3742]

Some discrepancies between platforms are to be expected, but this is a pretty big one. Being unable to start an task from inside startup() is a substantial enough limitation, especially given the recent deprecation of add_background_task(), so I think it's worth addressing.