Description

Continuation from #50597, applying the offset fix by default.

Migration guide

1. Set all instances of Scaffold.shouldSnackBarIgnoreFABRect to true after this PR is merged.
2. Fix all failing tests. There are likely two main types:
   a) Fix all golden tests to expect the new change.
   b) Fix any widget tests that expect the SnackBar to appear higher than it should. The difference should simply be the size of the floating action button's rect.
3. Once Scaffold.shouldSnackBarIgnoreFABRect is set to true by default (in a subsequent PR), remove the parameter from all instances of Scaffold.

Related Issues

Addresses #47202
Addresses #43716

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 read the Tree Hygiene wiki page, which explains my responsibilities.
• 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

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.
  • I wrote a design doc and migration guide: https://flutter.dev/go/floating-snackbar-offset