flutter Allow SliverMainAxisGroup children to remain pinned

Fix #178808.

Pre-launch Checklist

[X] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[X] I read the Tree Hygiene wiki page, which explains my responsibilities.
[X] I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
[X] I signed the CLA.
[X] I listed at least one issue that this PR fixes in the description above.
[x] I updated/added relevant documentation (doc comments with ///).
[X] I added new tests to check the change I am making, or this PR is test-exempt.
[X] I followed the breaking change policy and added Data Driven Fixes where supported.
[X] All existing and new tests are passing.

Nov 19 '25 16:11 manu-sncf

The push (d8aaeb6) is for migration from @manu-sncf to @zemanux

Dec 04 '25 08:12 zemanux

Hey @manu-sncf (and @zemanux)! Thanks for working on this. Is it possible that https://github.com/flutter/flutter/pull/179132 is accomplishing the same thing? We had a recent regression in behavior of pinned slivers in SliverMainAxisGroup, which that PR should fix.

Dec 10 '25 19:12 Piinks

The last commit b083b669b47e615576bd0f67406bcebb2098ed23 is only a rebase.

@Piinks, I’ve tried to provide as much context as possible here. I believe this PR complements PR #179132 well and will enable the creation of standalone sliver header components. Let me know what you think.

Dec 17 '25 08:12 zemanux