feat(plugin-autocapture-browser): add Min Page Y to viewport content updated

[jxiwang]

Summary

Adds [Amplitude] Min Page Y to [Amplitude] Viewport Content Updated so we can see where viewport tracking started. Linear: ZON-101.

[Amplitude] Max Page Y is the bottom of the furthest viewport (max scrollY + viewport height). Min Page Y is the top of the highest viewport seen since tracking began for the current page view: the smallest window.scrollY, including the offset at the moment the scroll tracker attaches. A non-zero value on the first event of a page view means the SDK started below the top of the page (late script injection, consent gating, scroll restoration, or a hash navigation).

The tracker also seeds max scroll from that same initial sample. Previously max stayed 0 until the next scroll event, so a late load under-reported [Amplitude] Max Page Y as only the viewport height.

On a new page view the range resets, then re-seeds once history has been applied (pushState / replaceState) or after popstate. Navigation API traversals wait for navigatesuccess, which runs after the browser restores scroll. Sampling inside the navigate handler copied the previous page's offset into the new page's Min and Max Page Y. An exposure snapshot that finishes before navigatesuccess waits for that sample, so a long intercept does not flush Min and Max Page Y as 0. If the navigation fails or is cancelled, navigateerror or the navigate event's abort signal releases that wait and samples the offset the page is still at. A change in the minimum flushes on the next Viewport Content Updated trigger, including page end.

The viewport exposure harness (test-server/autocapture/viewport-exposure.html) can defer SDK startup with ?deferInit=1. The Playwright spec scrolls before start(), then down and back up, and checks the sent Min Page Y and Max Page Y against those scroll positions. The same range is shown on the page.

Later checks lock the late-load cases the first spec did not:

• No scroll after attach: the first event's Min Page Y and the scroll component of Max Page Y are the offset where the SDK attached.
• Scrolling only upward after attach keeps Max Page Y at that offset.
• Movement from before the SDK attached is not reconstructed. A peak the visitor reached and left before init is absent, and a zone that was on screen then scrolled above the viewport is not backfilled into [Amplitude] Element Exposed.
• popstate re-seeds from the scroll position the browser has already restored, instead of carrying the previous page's max forward.
• A Navigation API traversal re-seeds from the scroll position restored before navigatesuccess. A scroll event that arrives before that success is the new page's baseline, not an extension of the previous page's max. A snapshot that finishes first waits for that success instead of sending 0. A failed or aborted navigation releases the same wait.

Checklist

• Does your PR title have the correct title format?
• Does your PR have a breaking change?: No

  

[linear-code]

SDK-355

[github-actions]

size-limit report 📦

Path	Size
packages/analytics-browser/lib/scripts/amplitude-min.js.gz	66.96 KB (+0.57% 🔺)
packages/session-replay-browser/lib/scripts/session-replay-browser-min.js.gz	135.88 KB (0%)
packages/unified/lib/scripts/amplitude-min.umd.js.gz	221.34 KB (+0.17% 🔺)
@amplitude/element-selector (gzipped esm)	3.48 KB (0%)

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