User menu causes content shift on mobile in Stacked Layout shell

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What component (if applicable)

• URL for category: https://tailwindcss.com/plus/ui-blocks/application-ui/application-shells/stacked
• Component name: stacked layout

Describe the bug
Opening the user menu on mobile in Stacked Layout briefly shows a scrollbar, causing content to jump horizontally

To Reproduce
Steps to reproduce the behavior:

1. Go to https://tailwindcss.com/plus/ui-blocks/application-ui/application-shells/stacked
2. Pick any stacked layout
3. Reduce width to force mobile layout
4. Open user menu
5. See scrollbar flash that makes content jump around