Description

After the change in #39535 we were only scheduling the first frame on accident as a side-effect of the warm-up frame (SchedulerBinding. scheduleWarmUpFrame awaits SchedulerBinding.endOfFrame, which implicitly schedules a frame). This PR makes scheduling the frame more explicit right after the root widget is attached (that's the first point in time where the framework can produce a meaningful frame).

Thanks to @brandondiamond for figuring this out and bringing it to my attention.

Related Issues

#39494

Tests

I added the following tests:

• Verify that attaching a root widget schedules a frame.

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