Description

This change lerps the width in a SizedBox that is used as spacing in the extended nav rail, rather than using a constant value. This fixes a bug in the animation of the extended rail, which also can cause inconsistencies with the leading/trailing widgets.

Before (slowed down 15x):

After fix (slowed down 15x):

Note that the change is subtle, but it is more apparent, and causes more issues, when there are animated widgets in the leading/trailing slots of the rail.

Related Issues

closes #65657

Tests

I added the following tests:

I added 'Extended rail transition does not jump from the beginning' to navigation_rail_test.dart

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: https://flutter.dev/go/template Replace this with a link to your design doc's short link
  • I got input from the developer relations team, specifically from: redbrogden
  • I wrote a migration guide: https://flutter.dev/go/breaking-changes-template Replace this with a link to a pull request that adds the migration guide to https://flutter.dev/docs/release/breaking-changes

Since the animation does not jump, the width of the rail is slightly different mid-animation, so one of the width values had to be updated.