Fixes https://github.com/flutter/flutter/issues/126096.
Fixes https://github.com/flutter/flutter/issues/126097.
Fixes https://github.com/flutter/flutter/issues/126102.
Fixes https://github.com/flutter/flutter/issues/126098.
Fixes https://github.com/flutter/flutter/issues/126147.
Work towards https://github.com/flutter/flutter/issues/126100.

Does not fix the OverlyEntry/ModalRoute leak (https://github.com/flutter/flutter/issues/126100).