chore: fix storybook dev mode warning

[TarunAdobe]

Description

Define a safe window.__swc shim in Storybook (storybook/preview.js) so debug warnings (window.__swc.warn) cannot crash the Storybook runtime.

Motivation and context

• When running storybook:build for 1st‑gen and serving the static Storybook docs, opening components that use overlays and SlottableRequestEvent would throw TypeError: Cannot read properties of undefined (reading 'warn').
• This happened because the dev build inlines window.__swc?.DEBUG to true, causing window.__swc.warn(...) to be called even when window.__swc is not defined in the Storybook runtime.
• The change introduces a Storybook‑only shim so debug warnings remain visible (via console.warn) without breaking stories, and allows us to keep the experimental warning in SlottableRequestEvent.

Related issue(s)

• fixes [Issue Number]

Screenshots (if appropriate)

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Author's checklist

• I have read the CONTRIBUTING and PULL_REQUESTS documents.
• I have reviewed at the Accessibility Practices for this feature, see: Aria Practices
• I have added automated tests to cover my changes.
• I have included a well-written changeset if my change needs to be published.
• I have included updated documentation if my change required it.

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Reviewer's checklist

• Includes a Github Issue with appropriate flag or Jira ticket number without a link
• Includes thoughtfully written changeset if changes suggested include patch, minor, or major features
• Automated tests cover all use cases and follow best practices for writing
• Validated on all supported browsers
• All VRTs are approved before the author can update Golden Hash

Manual review test cases

• Storybook build and static docs load without overlay errors

1. Go here
2. Navigate to stories that use sp-action-menu, sp-menu, and sp-picker or sp-overlay-trigger.
3. Open and interact with overlays (menus, pickers) and verify there is no Cannot read properties of undefined (reading 'warn') error in the console.

• Debug warning is emitted but does not crash

1. Go here
2. Open the browser devtools console.
3. Confirm that a warning about experimental slottable-request events appears, prefixed with [SWC], and that the story continues to function normally.

Device review

• Did it pass in Desktop?
• Did it pass in (emulated) Mobile?
• Did it pass in (emulated) iPad?

[changeset-bot]

⚠️ No Changeset found

Latest commit: 4ecbe3e

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[github-actions]

📚 Branch Preview Links

• Documentation Site (first-gen)
• Storybook (first-gen)
• Storybook (second-gen)

🔍 First Generation Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-5896

If the changes are expected, update the current_golden_images_cache hash in the circleci config to accept the new images. Instructions are included in that file.
If the changes are unexpected, you can investigate the cause of the differences and update the code accordingly.

[nikkimk]

lgtm

[marissahuysentruyt]

I think this looks fine- it also might fix a bug Helen raised that Casey and I chatted through today in slack! 🤞

I can confirm I see the dev warnings:

Separately, I'll make a ticket to dig into why the menu-item-with-description story isn't rendering.

Screen.Recording.2025-11-18.at.4.51.48.PM.mov

[marissahuysentruyt]

I always get tripped up with !!- can you explain this change for me?

[Rajdeepc]

@marissahuysentruyt They're functionally the same in this context. The !! pattern is useful when you need to store or return an explicit boolean, here its not the case. In this case, if (nextItem) already performs implicit boolean coercion, so the double negation adds no value. No impact on logic, just cleaner code.
Let me make it more clearer for you.

When !! matters:

// Storing as boolean (useful)
const hasNextItem: boolean = !!nextItem;

// Returning explicit boolean from function (useful)
function hasItem(): boolean {
  return !!nextItem;  // ensures boolean, not MenuItem | undefined
}

// API response where type matters (useful)
const response = { isActive: !!someValue };

When !! is redundant:

// Conditional check (unnecessary - if already evaluates truthiness)
if (!!nextItem) { ... }  // same as if (nextItem)
if (nextItem) { ... }     // already truthy check

// Boolean operators (unnecessary)
const result = !!condition && doSomething();  // same as condition && doSomething()