[maximilliangrand]

Checklist

• Includes tests
• Update documentation (changelog entry and existing Storybook assertions updated)

Changes proposed in this pull request:

A collapsible Section currently exposes aria-expanded="false" when its content is open and "true" when closed. This contradicts both the visible state and the control's accessible name.

Use !isCollapsed for aria-expanded, matching the state passed to Collapse and the ARIA disclosure pattern. Correct the existing unit and Storybook expectations, and cover mouse/Enter/Space activation plus controlled updates.

Reviewers should focus on:

The expanded state should follow the rendered content. In controlled mode, invoking onToggle should leave the state unchanged until isOpen changes.

Validation on macOS, Node 26.7.0:

• Section Vitest suite: 5 passed / 9 failed before, 14 passed after.
• Core test TypeScript check: passed.
• ESLint for the component and unit tests, Prettier for all changed files, and git diff --check: passed.
• Storybook play tests and the full suite were not run. Type-aware lint including the story exceeded an imposed 512 MiB heap limit; its three expectation changes were reviewed separately.

AI assistance was used for implementation, tests, and review.

Screenshot

No visual change. This corrects the control's exposed expanded state.

[palantirtech]

Thanks for your interest in palantir/blueprint, @maximilliangrand! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[changelog-app]

Generate changelog in packages/core/changelog/@unreleased

Type (Select exactly one)

• Feature (Adding new functionality)
• Improvement (Improving existing functionality)
• Fix (Fixing an issue with existing functionality)
• Break (Creating a new major version by breaking public APIs)
• Deprecation (Removing functionality in a non-breaking way)
• Migration (Automatically moving data/functionality to a new system)

Description

[core] Correct the expanded state exposed by Section

Check the box to generate changelog(s)

• Generate changelog entry