fix(Select/SelectMenu/InputMenu): add fallback for max-height

[benjamincanac]

🔗 Linked issue

Closes #6490

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

The max-h on popper content relied on --reka-*-content-available-height which chains to --reka-popper-available-height. floating-ui sets that variable asynchronously, so on the initial render it's undefined — the min() expression becomes invalid and max-height drops to none. Without a bounded container the virtualizer renders every item at once.

Adds 15rem as the var() fallback so the constraint holds before floating-ui positions the element, matching the original fixed max-h-60.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds CSS variable fallbacks to three theme files (input-menu.ts, select-menu.ts, and select.ts) that define dropdown and menu content styling. Each file's slots.content class string is updated to use explicit fallback values (15rem) for max-height CSS variables when unset. Additionally, select-menu.ts includes an origin-(--reka-combobox-content-transform-origin) class for transform origin control. The changes standardize dropdown content height computation across the UI components.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains the bug, root cause (async floating-ui), impact on virtualization, and the solution with fallback values.
Linked Issues check	✅ Passed	The PR successfully addresses issue #6490 by adding CSS variable fallbacks (15rem) to restore max-height constraints before floating-ui positions elements, enabling virtualization.
Out of Scope Changes check	✅ Passed	All changes are scoped to three theme files and directly address the regression fix by adding CSS variable fallbacks for max-height.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title accurately describes the main change: adding fallback values for max-height in CSS variables across three theme files (Select, SelectMenu, InputMenu).

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/select-menu-virtualize-regression

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[pkg-pr-new]

npm i https://pkg.pr.new/@nuxt/ui@6503

commit: 4fa3e59