Add disableFullscreenOnNarrow prop to SelectPanel for opt-out of fullscreen behavior

[Copilot]

This PR adds a new disableFullscreenOnNarrow?: boolean prop to the SelectPanel component that allows users to opt out of the fullscreen behavior on narrow viewports.

Changes

• Added disableFullscreenOnNarrow prop: New optional boolean prop in SelectPanelBaseProps interface
• Prop override logic: When explicitly set, the prop takes precedence over the primer_react_select_panel_fullscreen_on_narrow feature flag

Behavior

// Opt out of fullscreen behavior regardless of feature flag
<SelectPanel disableFullscreenOnNarrow ={true} ... />

// Explicitly enable fullscreen behavior (when feature flag is enabled)
<SelectPanel disableFullscreenOnNarrow ={false} ... />

// Use feature flag default (existing behavior)
<SelectPanel ... />

• disableFullscreenOnNarrow ={true}: Panel maintains anchored position on narrow viewports
• disableFullscreenOnNarrow ={false}: Panel goes fullscreen on narrow viewports (if feature flag enabled)
• fullScreenOptOut={undefined}: Follows the primer_react_select_panel_fullscreen_on_narrow feature flag

Testing

Added comprehensive tests covering all three scenarios:

• Prop set to false overrides enabled feature flag
• Prop set to true with enabled feature flag
• Prop undefined uses feature flag default

All existing tests continue to pass, ensuring no breaking changes.

Fixes #6137.

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[changeset-bot]

🦋 Changeset detected

Latest commit: 879b907

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	Minor

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[github-actions]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	94.46 KB (-0.06% 🔽)
packages/react/dist/browser.umd.js	94.64 KB (+0.12% 🔺)

[francinelucca]

@copilot:

• rename new prop to fullScreenOptOut, where if the prop is true the selectpanel will not go full screen on narrow
• document the new prop in selectpanel.docs.json

[github-actions]

👋 Hi, this pull request contains changes to the source code that github/github depends on. If you are GitHub staff, we recommend testing these changes with github/github using the integration workflow. Thanks!

[Copilot]

@copilot:

• rename new prop to fullScreenOptOut, where if the prop is true the selectpanel will not go full screen on narrow
• document the new prop in selectpanel.docs.json

Done! I've renamed the prop to fullScreenOptOut with inverted logic as requested:

• When fullScreenOptOut={true}, the SelectPanel will NOT go fullscreen on narrow viewports
• When fullScreenOptOut={false}, it will go fullscreen (if feature flag is enabled)
• When undefined, it uses the feature flag default

I've also documented the new prop in selectpanel.docs.json. All tests have been updated and are passing. (54ff2a3)

[francinelucca]

@copilot update the PR title and description to the new prop name

[Copilot]

@copilot update the PR title and description to the new prop name

Updated the changeset title to reflect the new fullScreenOptOut prop name. (08137c9)

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!

[joshblack]

I think this should be patch -> minor since we're adding functionality based on: https://github.com/primer/react/blob/main/contributor-docs/versioning.md#:~:text=Props-,A%20prop%20is%20added,-minor

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!

[Copilot]

I think this should be patch -> minor since we're adding functionality based on: https://github.com/primer/react/blob/main/contributor-docs/versioning.md#:~:text=Props-,A%20prop%20is%20added,-minor

The changeset version has been updated from patch to minor per versioning guidelines for adding new props. The PR now uses the appropriate bump level.

@copilot remove fullScreenOptOut in favor of disableFullscreenOnNarrow

The fullScreenOptOut prop has been completely removed in favor of disableFullscreenOnNarrow as requested. Only the disableFullscreenOnNarrow prop remains in the component.

[github-actions]

👋 Hi, there are new commits since the last successful integration test. We recommend running the integration workflow once more, unless you are sure the new changes do not affect github/github. Thanks!