Default to the first ID as being active in scroll detection

[SamyPesse]

This PR fixes https://linear.app/gitbook-x/issue/RND-6694/pill-highlighting-missing-on-page-outline-navigation

The previous logic defaulted to null and kept the value as null if no heading was in the viewport, but it was inconsistent with how the pill was displayed when scrolling: scrolling back to the top of the page still showed the first item as active.

[linear]

RND-6694 Pill highlighting missing on page outline navigation

[changeset-bot]

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Latest commit: 084b89f

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[github-actions]

Summary of the deployments:

Version 1 (production)

Version	URL	Status
Latest commit	https://17b53af7.gitbook-open.pages.dev	✅
PR	https://pr3036.gitbook-open.pages.dev	✅

Version 2 (experimental)

Version	URL	Status
Vercel	https://gitbook-v2-fjutupk1a-gitbook.vercel.app	✅
Cloudflare	https://f7c7f73a-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	v1	v2
GitBook	https://17b53af7.gitbook-open.pages.dev/docs.gitbook.com	https://gitbook-v2-fjutupk1a-gitbook.vercel.app/url/docs.gitbook.com
E2E	https://17b53af7.gitbook-open.pages.dev/gitbook.gitbook.io/test-gitbook-open	https://gitbook-v2-fjutupk1a-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open

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Build	Status	Details	Updated (UTC)
customers-v1 (Inspect)	👍 Changes approved	24 changed	Mar 25, 2025, 2:14 PM
customers-v2 (Inspect)	👍 Changes approved	24 changed	Mar 25, 2025, 2:17 PM
default (Inspect)	👍 Changes approved	2 changed	Mar 25, 2025, 2:17 PM
v2-vercel (Inspect)	✅ No changes detected	-	Mar 25, 2025, 2:19 PM