SliverFillRemaining accounts for child size when hasScrollBody is false - #35810

Description

SliverFillRemaining stipulates that it will size its child to fill the remaining space in the viewport in the main axis.
Its actual default behavior has been to fill the entire viewport in the main axis, which is desirable behavior for applications such as NestedScrollView, but is not the expected behavior for most cases.

#33627 added the flag hasScrollBody, which allows developers to specify if the child is a scrollable, like in a NestedScrollView, or not. However, this implementation did not take into account cases when the child's size should be used instead of the calculated extent of the sliver.
This PR modifies the original implementation of #33627 to include this consideration.
When hasScrollBody is false, the child's size will be compared to that of the calculated extent to choose the appropriate size.

Also in this PR is an additional flag - fillOverscroll. This give developers even more power over the layout of this widget, as they can specify what the behavior should be for scroll physics that utilize an overscroll feature, such as with the default scroll behavior on iOS platforms. fillOverscroll defaults to false, meaning the child widget will maintain its size during overscroll. When set to true, it will cause the child's size to stretch and fill the overscroll area.

An executable study app is available here that illustrates use cases and behaviors: gist

Related Issues

Fixes #35758
Related to #35753, will follow-up with documentation and diagrams.
Related to #17444 (#33627)

Tests

SliverFillRemaining - hasScrollBody

• does not extend past viewport when false
• scrolls beyond viewport by default
• child without size is sized by extent when false
• child with size is sized by extent when false
• extent is overridden by child with larger size when false
• extent is overridden by child size if precedingScrollExtent > viewportMainAxisExtent when false

iOS/similar scroll physics when hasScrollBody: false & fillOverscroll: true

• child without size is sized by extent and overscroll
• child with size is overridden and sized by extent and overscroll
• extent is overridden by child size and overscroll if precedingScrollExtent > viewportMainAxisExtent

Android/other scroll physics when hasScrollBody: false, ignores fillOverscroll: true

• child without size is sized by extent, fillOverscroll is ignored
• child with size is overridden and sized by extent, fillOverscroll is ignored
• extent is overridden by child size if precedingScrollExtent > viewportMainAxisExtent, fillOverscroll is ignored

Checklist

Before you create this PR confirm that it meets all requirements listed below by checking the relevant checkboxes ([x]). This will ensure a smooth and quick review process.

• 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.