`--available-height` unexpectedly changes when opening keyboard on iOS

# Bug report

## Current behavior

This is testing the following example on iOS: https://base-ui.com/react/components/combobox#input-inside-popup

https://github.com/user-attachments/assets/a6e32228-977b-4cd2-bd66-ab6dc68ee6c4

## Expected behavior

When the filter input is focused and the keyboard opens (or vice versa), the menu should change size.

## Reproducible example

https://base-ui.com/react/components/combobox#input-inside-popup

## Base UI version

1.0.0-beta.4

## Which browser are you using?

Safari iOS

## Which OS are you using?

iOS

## Which assistive tech are you using (if applicable)?

N/A

## Additional context

I'm only guessing this is because of `--available-height`. I could be wrong.