Revert "perf(TreeView): replace O(n) TreeWalker with O(depth) sibling traversal"

[francinelucca]

Reverts #7544

Found failing checks in integration testing, reverting

[changeset-bot]

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Latest commit: 5414e86

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[github-actions]

⚠️ Action required

👋 Hi, this pull request contains changes to the source code that github/github-ui depends on. If you are GitHub staff, test these changes with github/github-ui using the integration workflow. Check the integration testing docs for step-by-step instructions. Or, apply the integration-tests: skipped manually label to skip these checks.

[Copilot]

Pull request overview

This PR reverts PR #7544, which replaced the TreeWalker-based O(n) navigation in getVisibleElement with an O(depth) sibling traversal approach. The revert was triggered by failing integration tests discovered after the original PR was merged.

Changes:

• Restores the original TreeWalker-based implementation of getVisibleElement in useRovingTabIndex.ts, removing all the helper functions (getNextVisibleElement, getPreviousVisibleElement, getDeepestLastDescendant, etc.) that were introduced in #7544.
• Removes the test asserting that collapsed subtree children are not rendered in the DOM, which was added alongside the sibling traversal approach.
• Deletes the changeset file for the reverted change.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/react/src/TreeView/useRovingTabIndex.ts	Reverts getVisibleElement back to the TreeWalker-based implementation
packages/react/src/TreeView/TreeView.test.tsx	Removes the test for collapsed subtree DOM rendering added in #7544
.changeset/treeview-sibling-traversal.md	Deletes the changeset for the reverted change

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[primer-integration]

👋 Hi from github/github-ui! Your integration PR is ready: https://github.com/github/github-ui/pull/15939

[primer-integration]

Integration test results from github/github-ui:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!