Extending fillOverscroll Feature for SliverFillRemaining - #45121

Description

This PR extends the fillOverscroll feature of SliverFillRemaining to enable solely filling the overscroll area, rather than also having a child in the natural un-overscrolled scroll view.

This use case applies when SliverFillRemaining's child has no size and the precedingScrollExtent has gone beyond the size of the viewportMainAxisExtent. Prior to this change, this would result in nothing being drawn by SliverFillRemaining. Now it will draw into the overscroll area as it appears. :)

Note:
This PR contains changes from #44471, which should land first.
I branched from there to prevent a self-inflicted merge conflict. :)

Related Issues

Fixes #44353

Tests

• fillOverscroll works when child has no size and precedingScrollExtent > viewportMainAxisExtent

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• No, this is not a breaking change.