fix(core): remove unused source preview anchor

[RovshanMuradov]

Summary

Remove the unused source-preview CSS anchor to avoid excessive Chromium layout work with many math previews.

Fixes #3098.

Rationale

Every preview declares --bn-source-popup-anchor, but no current position-anchor, anchor(), or anchor-size() consumer uses it. Source popups use absolute positioning. Removing the leftover declaration avoids registering hundreds of unnecessary anchors; retaining it only while open adds state-dependent CSS without an internal consumer.

Changes

• Delete anchor-name: --bn-source-popup-anchor from .bn-preview-container.

Impact

For 500 inline formulas, median cumulative layout falls from 17.610 → 0.437 s editable and 17.684 → 0.427 s read-only. Formula counts and DOM node counts are unchanged.

The anchor identifier is undocumented; downstream CSS explicitly targeting it would need adjustment.

Testing

• Compared original, active-only, and removal production fixtures built from current source: 72 serial Chromium navigations, three runs per combination, 100/250/500 inline formulas and 100 block formulas, editable/read-only. No page errors. Both fixes eliminate the slowdown; no meaningful performance difference between them was established.
• Chromium/Firefox standalone functional comparisons: inline/block math and a custom SourceBlockWithPreview, edit persistence, OK/Escape, popup switching, keyboard/focus, editable transitions, two-editor focus switching, scrolling, and narrow viewport. Recorded geometry, document content, and state match the original.
• CSS formatting and git diff --check pass.
• pnpm lint run as requested by CONTRIBUTING: 54 type errors and one warning on both the unchanged base and this patch, with identical diagnostic messages (including missing @blocknote/xl-typst-compiler declarations).

Screenshots/Video

Not applicable; no intended visual change. Reproduction and measurements are in #3098.

Checklist

• Code follows the project's coding standards.
• Unit tests covering the new feature have been added.
• All existing tests pass.
• The documentation has been updated to reflect the new feature

Additional Notes

This is a one-line CSS cleanup with standalone browser verification; no new unit tests or public API documentation changes. The full monorepo build/unit/Docker E2E suites were not run. WebKit verification was blocked by missing host libraries.

Summary by CodeRabbit

• Bug Fixes
  • Removed an obsolete anchor declaration from the preview container while preserving its clickable behavior.

[vercel]

@RovshanMuradov is attempting to deploy a commit to the TypeCell Team on Vercel.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Advanced

Run ID: 9ad26edc-3f0d-4f82-8350-e16272185cd0

📥 Commits

Reviewing files that changed from the base of the PR and between 835fabf and 7362201.

📒 Files selected for processing (1)

• packages/core/src/editor/Block.css

💤 Files with no reviewable changes (1)

• packages/core/src/editor/Block.css

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

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📝 Walkthrough

Walkthrough

The change removes the unused CSS Anchor Positioning declaration from .bn-preview-container. The container remains clickable through its existing pointer cursor.

Changes

Preview layout performance

Layer / File(s)	Summary
Remove unused preview anchor packages/core/src/editor/Block.css	.bn-preview-container no longer declares anchor-name: --bn-source-popup-anchor. Its cursor: pointer declaration remains.

Priority: ➖ Normal

Estimated code review effort: 1 (Trivial) | ~3 minutes

Change: Bug fix · Severity of issue fixed: Medium

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the primary change: removing the unused source preview anchor declaration.
Description check	✅ Passed	The description follows the required template and clearly documents the rationale, code change, impact, testing, limitations, and checklist status.
Linked Issues check	✅ Passed	Issue #3098 requires preventing unused CSS-anchor layout work for .bn-preview-container source previews. The reviewed change removes anchor-name: --bn-source-popup-anchor from `packages/core/src/e…
Out of Scope Changes check	✅ Passed	The whole-PR summary identifies one scoped change in packages/core/src/editor/Block.css: removal of the unused anchor declaration. The reported validation supports the performance objective in issue…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…

✨ Finishing Touches 💡 1

🛠️ Fix failing CI checks 💡

• Commit to this branch
• Create a new PR

🧪 Generate unit tests (beta)

• Create a new PR

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_{A rabbit finds an anchor unused
It hops away, no layout bruised
The preview still points with care
While lighter calculations fill the air
Math previews bloom, swift and bright}

_{Comment @coderabbitai help to get the list of available commands.}