SPFx 1.22.x / 1.23 throws Cannot read properties of undefined (reading 'set') in debug mode when using Fluent UI React v9 components

Target SharePoint environment

SharePoint Online

What SharePoint development model, framework, SDK or API is this about?

💥 SharePoint Framework

Developer environment

Windows

What browser(s) / client(s) have you tested

• 💥 Internet Explorer
• 💥 Microsoft Edge
• 💥 Google Chrome
• 💥 FireFox
• 💥 Safari
• mobile (iOS/iPadOS)
• mobile (Android)
• not applicable
• other (enter in the "Additional environment details" area below)

Additional environment details

• browser version 148.0.7778.217
• SPFx version 1.22.x & 1.23
• Node.js version 22.14.0
  -Fluent UI 9 - Tested with 9.56.6 - 9.66.6, 9.74.1

Describe the bug / error

When the web part is loaded from localhost, the following exception is thrown in the browser console:

TypeError: Cannot read properties of undefined (reading 'set')

Stack trace:

TypeError: Cannot read properties of undefined (reading 'set')
    at getModalizer (https://localhost:4321/dist/promotion-portal-web-part.js:96165:34)
    at initTabsterModules (https://localhost:4321/dist/promotion-portal-web-part.js:34449:58)
    at https://localhost:4321/dist/promotion-portal-web-part.js:34673:40
    at https://res-1.public.onecdn.static.microsoft/files/sp-client/sp-pages-assembly_en-us_*.js

The stack trace consistently points to Tabster initialization (getModalizer / initTabsterModules).

Steps to reproduce

1. Create an SPFx web part using SPFx 1.22.2 or 1.23.
2. Install Fluent UI React v9 (@fluentui/react-components).
3. Use Fluent UI v9 controls within the web part.
4. Run the solution using: npm run start
5. Open the SharePoint page that hosts the web part.

Additional Information

• Reproduced in SPFx 1.22.2 and 1.23.
• Reproduced with multiple Fluent UI React v9 versions.
• Only observed when running from localhost in debug mode.
• The issue appears related to Tabster initialization which was previously working fine.

Expected behavior

Fluent UI React v9 components should work without throwing getModalizer / initTabsterModules errors, even when running in local debug mode