Fix section group dropdown offset on big screens

[zenoachtig]

I introduced a layout bug in #3655, this fixes it.

We calculate the trigger's position on the screen and set the dropdown menu to be below that. But because of position: absolute it'll be constrained to the nearest relative parent, which is the site's container. This is not an issue when the viewport is smaller than the max container width, but introduces offset from the trigger on bigger displays.

Setting the position to fixed solves this.

[changeset-bot]

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Latest commit: 3c2ef87

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-rc6fifq7m-gitbook.vercel.app	✅
Cloudflare	https://85e1e3c2-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-rc6fifq7m-gitbook.vercel.app/url/gitbook.com/docs	https://85e1e3c2-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-rc6fifq7m-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://85e1e3c2-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.gitbook.io/test-gitbook-open

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Build	Status	Details	Updated (UTC)
customers-v2 (Inspect)	👍 Changes approved	5 changed	Sep 17, 2025, 4:56 PM
v2-cloudflare (Inspect)	✅ No changes detected	-	Sep 17, 2025, 5:01 PM
v2-vercel (Inspect)	✅ No changes detected	-	Sep 17, 2025, 4:14 PM