[select] Check for 100% max-height style

[atomiks]

• I have followed (at least) the PR section of the contributing guide.

The "old" JSX structure without <Select.List> regressed from #3573, but this is still supported. The max-height was calculated as 100% due to the inline style, which we can check for.

https://deploy-preview-3749--base-ui.netlify.app/experiments/long-select

[pkg-pr-new]

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@3749
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@3749
  

commit: 2ead2c6

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+32B(+0.01%)	🔺+18B(+0.01%)

Details of bundle changes

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[greptile-apps]

Greptile Summary

This PR fixes a regression in the select component's alignItemWithTrigger feature for the "old" JSX structure (without Select.List). The issue was introduced in PR #3573.

Root Cause:
When alignItemWithTrigger is active, line 316 sets an inline style popupElement.style.height = '100%'. The getMaxPopupHeight function was calling parseFloat() on this value, which returned NaN for the string '100%'.

Solution:
The fix explicitly checks if maxHeightStyle === '100%' and returns Infinity in that case, which correctly indicates no maximum height constraint. This ensures proper height calculations for both JSX structures (with and without Select.List).

Changes:

• Modified getMaxPopupHeight to accept CSSStyleDeclaration instead of HTMLElement
• Added explicit check for '100%' value before parsing
• Updated both call sites to pass computed styles instead of element

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The fix is a targeted, well-reasoned change that addresses a specific regression. The logic is sound: explicitly handling the '100%' case before parseFloat prevents NaN issues. The change maintains backward compatibility with both JSX structures (with/without Select.List), and the fix aligns with the component's existing inline style usage on line 316.
• No files require special attention

Important Files Changed

Filename	Overview
packages/react/src/select/popup/SelectPopup.tsx	Fixed regression for old JSX structure without Select.List by checking for 100% max-height inline style

Sequence Diagram

sequenceDiagram
    participant User
    participant SelectPopup
    participant getMaxPopupHeight
    participant Browser

    User->>SelectPopup: Open select with alignItemWithTrigger active
    SelectPopup->>Browser: Set popupElement.style.height = '100%'
    SelectPopup->>Browser: getComputedStyle(popupRef.current)
    Browser-->>SelectPopup: CSSStyleDeclaration
    SelectPopup->>getMaxPopupHeight: Call with popupStyles
    getMaxPopupHeight->>getMaxPopupHeight: Check if maxHeightStyle === '100%'
    alt maxHeightStyle is '100%'
        getMaxPopupHeight-->>SelectPopup: Return Infinity
    else maxHeightStyle is numeric
        getMaxPopupHeight->>getMaxPopupHeight: parseFloat(maxHeightStyle)
        getMaxPopupHeight-->>SelectPopup: Return parsed value or Infinity
    end
    SelectPopup->>SelectPopup: Calculate viewportHeight using maxPopupHeight
    SelectPopup->>Browser: Apply positioning and height styles

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