fix(extension): only remove highlights we register

[danielroe]

at the moment, pages that highlight their own content with the CSS Custom Highlight API lose that highlighting when Margin is enabled.

I spotted this when visiting https://davatron5000.github.io/microlighter/ in Firefox. the code samples highlight for a moment, then go plain and stay plain, and [...CSS.highlights] on the page is empty afterwards.

Screen.Recording.2026-09-06.at.13.16.37.mov

(it doesn't reproduce if I disable the margin extension)

I think the issue is that we unilaterally clear all highlights, rather than just what we register... (the registry is per-Document and shared with the page)

so, this PR tracks the names we register and deletes just those

Summary by CodeRabbit

• Bug Fixes
  • Prevented the extension from clearing highlights created by other page features.
  • Improved highlight cleanup when the overlay is disabled, refreshed, or the page URL changes.
  • Preserved scroll-to-text and badge highlighting while ensuring only extension-owned highlights are removed.

[coderabbitai]

📝 Walkthrough

Walkthrough

The overlay now tracks its owned CSS highlights through shared helpers. Disable, re-render, and URL-change flows clear only those highlights. Scroll-to-text and badge highlights use the ownership-aware registration and removal methods.

Changes

Owned highlight lifecycle

Layer / File(s)	Summary
Highlight ownership helpers extension/src/utils/overlay.ts	The overlay tracks owned highlight names and routes scroll-to-text highlight creation and removal through shared helpers.
Highlight lifecycle integration extension/src/utils/overlay.ts	Disable, badge re-render, and URL-change flows clear only owned highlights. Badge highlights use the shared registration helper.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: 🟡 Moderate · up to 2efcb

The change stops broad highlight clearing, but pages that use a colliding highlight name can still have their current highlight replaced or removed during badge refresh, disable, URL changes, or delayed scroll cleanup. Preserve and conditionally restore registry entries before merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: the extension removes only highlights that it registered.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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Inline comments:
In `@extension/src/utils/overlay.ts`:
- Around line 62-65: Update setHighlight and the related cleanup logic to track
each name’s original CSS.highlights entry and the exact registered Highlight
object, rather than names alone. During cleanup, restore the original entry only
when the registry still contains the tracked object; otherwise leave the current
entry unchanged, and bind delayed scroll cleanup to that registered object. Add
regression coverage for scroll, disable/re-render, and URL-change cleanup.

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ℹ️ Review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 6285315 and 2efcb5e.

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• extension/src/utils/overlay.ts

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