Font size increases when orientation switches to horizontal

[randomguy-2650]

What version of starlight are you using?

0.37.0 (Starlight site)

What version of astro are you using?

5.6.7 (Starlight site)

What package manager are you using?

pnpm (Starlight site)

What operating system are you using?

iOS (26.1)

What browser are you using?

Safari or any mobile WebKit browser on iOS (basically all)

Describe the Bug

This is a bug about iOS’ screen rotation or orientation lock feature. If you unlock it and turn your device horizontally, it will increase the font size of elements for no reason, but only random elements in the sidebar, like the sidebar items in the <details>. But if you rotate it to vertical again, it does revert the change. Only when the sidebar is open, that it does not revert it.

Video:

ScreenRecording_12-06-2025.07-30-52_1.mp4

Expected result: Font size shouldn’t change. But if it does, it should revert back in vertical orientation.
Actual result: Font size increases permanently in horizontal orientation and doesn’t revert until refresh.

Link to Minimal Reproducible Example

https://starlight.astro.build/getting-started/ as an example, any page will work

Participation

• I am willing to submit a pull request for this issue.

[HiDeoo]

Thanks for the report 🙌

I can also reproduce this issue on my end. My first guess would be that it's related to what is usually called font boosting or text autosizing. I quickly tried to disable it using the following CSS:

-webkit-text-size-adjust: 100%;
text-size-adjust: 100%;

And it seems to have fixed the problem for me. Even tho MDN doesn't mention it's supported by Safari, it's definitely documented by Apple.

If someone with an Apple device is willing to test this fix more thoroughly, confirm it fixes the issue properly (without any downsides) and want to submit a PR, I would be happy to review it 👍