Fix AnchorNav layout shift when sticky #1124
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Pull Request Overview
This PR fixes a layout shift issue in the AnchorNav component when it becomes sticky by adding a spacer element to compensate for the lost height.
- Adds a height-compensating spacer element that appears only when the nav is sticky
- Updates tests to cover the new spacer behavior with proper mocking of IntersectionObserver and scroll events
- Fixes two broken stories by adding top offset to prevent the AnchorNav from being too close to the page top
Reviewed Changes
Copilot reviewed 4 out of 16 changed files in this pull request and generated 3 comments.
| File | Description |
|---|---|
| AnchorNav.tsx | Adds navHeight state tracking and conditional spacer element to prevent layout shift |
| AnchorNav.test.tsx | Comprehensive test setup for intersection observer and scroll behavior, plus test for spacer functionality |
| AnchorNav.features.stories.tsx | Adds top padding to stories to fix positioning issues |
| tame-boxes-shout.md | Changeset documenting the layout shift fix |
joshfarrant
approved these changes
Aug 1, 2025
Merged
This was referenced Aug 1, 2025
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Summary
Resolves https://github.com/github/primer/issues/5353
Fixes a bug where AnchorNav didn't compensate for lost height when it becomes sticky. This caused noticeably jumpy layout shift underneath the sticky element.
List of notable changes:
What should reviewers focus on?
Steps to test:
Supporting resources (related issues, external links, etc):
Contributor checklist:
update snapshotslabel to the PR)Reviewer checklist:
Screenshots:
Fixed in Storybook:
Screen.Recording.2025-07-30.at.13.43.13.mov
Screen.Recording.2025-07-30.at.13.42.46.mov
Fixed in Dotcom:
Screen.Recording.2025-07-31.at.16.09.04.mov
Screen.Recording.2025-07-31.at.16.08.33.mov
Note
In dotcom there is an additional x-axis shift when anchor nav becomes sticky. This is because the AnchorNav is wrapped in a Section component, which includes a max-width that is no longer applied when the nav becomes stuck. AnchorNav isn't intended to be used inside a grid or width-constrained container. Once that's removed, it will behave as-seen in the video above. This PR won't address this. It instead fixes the bug causing vertical layout shift. cc/ @seangolob