Fix Avatar sizing in Button leadingVisual slot

[Copilot]

Fixes #7974

Avatars passed into Button's leadingVisual slot render at the wrong width. Global img { max-width: 100% } collides with the less-specific Avatar width (set via CSS var), and because Button uses min-content for the leadingVisual grid area, max-width can resolve to 0, shrinking the image.

Changelog

New

Changed

• Avatar.module.css: set min-width alongside width (base rule + all responsive breakpoints) so Avatars never shrink below their intended size.
• AvatarStack.module.css: set min-width: var(--avatar-stack-size) on .AvatarItem. The Avatar min-width resolves to each child's inline --avatarSize-regular, so without this override custom-size children rendered at their individual sizes instead of the uniform stack size.

Removed

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

Verify via the Button-with-Avatar leadingVisual case (Avatar retains its expected width) and the AvatarStack CustomSizeOnChildren story (children remain uniformly sized at the smallest child's size).

Merge checklist

• Added/updated tests
• Added/updated documentation
• Added/updated previews (Storybook)
• Changes are SSR compatible
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge
• (GitHub staff only) Integration tests pass at github/github-ui (Learn more about how to run integration tests)

[changeset-bot]

🦋 Changeset detected

Latest commit: a06a00a

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

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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.

To publish a canary release for integration testing, apply the Canary Release label to this PR.

[Copilot]

Pull request overview

This PR addresses a layout bug where Avatar images can collapse to an incorrect width when rendered inside Button’s leadingVisual slot (due to min-content sizing interacting with a global img { max-width: 100% } rule). The fix makes Avatars maintain their intended rendered width by enforcing a matching min-width.

Changes:

• Add min-width alongside width in the base .Avatar rule.
• Add min-width for each responsive breakpoint (narrow/regular/wide) under data-responsive.
• Add a patch changeset documenting the user-visible fix.

Show a summary per file

File	Description
packages/react/src/Avatar/Avatar.module.css	Adds min-width to prevent Avatar images collapsing under constrained grid sizing contexts.
.changeset/avatar-min-width.md	Patch changeset describing the Avatar sizing fix (notably for Button leadingVisual).

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 1

[primer-integration]

👋 Hi from github/github-ui! Your npm packages integration PR is ready: https://github.com/github/github-ui/pull/23392.

Will report CI results once ready.