Support custom BoxBorder animation in BoxDecoration

[gabrimatic]

Fixes #180297

BoxDecoration now preserves the existing specialized Border and BorderDirectional interpolation path, while custom BoxBorder subclasses use ShapeBorder.lerp. This lets custom borders provide their own interpolation hooks and avoids the AnimatedContainer assertion when animating a BoxDecoration with a custom border.

Tests:

• ./bin/flutter test packages/flutter/test/painting/box_decoration_test.dart
• ../../bin/dart analyze lib/src/painting/box_decoration.dart test/painting/box_decoration_test.dart

[gemini-code-assist]

Code Review

This pull request enables BoxDecoration to handle interpolation and scaling for custom BoxBorder subclasses by introducing a private _lerpBorder method that utilizes ShapeBorder.lerp. New tests verify this functionality using a custom border implementation. Feedback suggests simplifying nullable type checks and renaming a local variable to avoid shadowing and improve idiomatic Dart usage.

[Piinks]

Thanks for the update!
WDYT about staying consistent with the implementation of ShapeBorder.lerp and OutlinedBorder.lerp? Should follow the same pattern for the fallback? This could also avoid the is check, which aligns better with the Flutter style guide.

What do you think about simplifying the interpolation logic to this one-liner?

final ShapeBorder? result = b?.lerpFrom(a, t) ?? a?.lerpTo(b, t);
return result as BoxBorder? ?? (t < 0.5 ? a : b);

This provides a resilient fallback and keeps the implementation consistent with the rest of the library. Let me know what you think!

[Piinks]

Thanks for addressing the feedback, this LGTM

[auto-submit]

autosubmit label was removed for flutter/flutter/186348, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

[justinmc]

Google are legit but may just be slight color changes.

[elliette]

Google are legit but may just be slight color changes.

Looks like these are just very slight changes to borders, therefore I've overridden the Google testing status.

[Piinks]

Re-LGTM