Update AvatarStack to not reserve extra layout space when collapsed

[llastflowers]

Closes https://github.com/github/primer/issues/6922

Changelog

Changed

Update AvatarStack &:nth-child(n + 6) CSS block to include position: absolute so that the hidden Avatars don't reserve layout space when collapsed (including when disableExpand is true).

Demo:

Screen.Recording.2026-08-03.at.12.19.38.mov

(Temporary story used for testing/demo only; not added in this PR.)

Rollout strategy

• Patch release
• Minor release
• Major release; if selected, include a written rollout or migration plan
• None; if selected, include a brief description as to why

Testing & Reviewing

Reviewers, please let me know if you foresee any potential issues (a11y or otherwise) with this solution. I could not think of any, and testing didn't reveal anything.

[changeset-bot]

🦋 Changeset detected

Latest commit: 154cbc4

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[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 pull request updates AvatarStack’s collapsed styling so hidden avatars don’t reserve layout space, aligning the component’s rendered layout more closely with what users see (including when disableExpand is enabled).

Changes:

• Switches the “avatars after the 5th” collapsed rule from visibility: hidden to display: none to prevent reserved space.
• Ensures hidden avatars can be re-shown on hover/focus-within by explicitly setting display in the expanded interaction styles.
• Adds a changeset for a patch release of @primer/react.

Show a summary per file

File	Description
packages/react/src/AvatarStack/AvatarStack.module.css	Updates collapsed/expanded CSS so extra avatars don’t reserve layout space when collapsed.
.changeset/fresh-ideas-worry.md	Adds a patch changeset documenting the user-facing behavior change.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 2
• Review effort level: Lite

[primer-integration]

Integration test results from github/github-ui PR:

  CI Passed
  VRT Passed
  Projects Passed

All checks passed!