fix(pixel): drop synthetic above-fold scroll_depth event (SDK-275)#2869
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The previous fix (v0.1.1) used a ResizeObserver to detect when the document grew beyond the viewport, but the synthetic above-fold event still produced false positives on SPAs and pages with internal scroll containers — where document.documentElement.scrollHeight is permanently equal to window.innerHeight regardless of actual content depth. Confirmed in the wild on godsunchained.com, which uses an internal .website-content scroll container with overflow: auto. Drop the synthetic event entirely and align with PostHog / Segment / GTM behaviour: only fire scroll_depth on real scroll events. On pages where the document doesn't scroll, no milestone fires. This trades the bounce-filter signal on legitimately short pages for eliminating an entire class of false positives. Net bundle impact: -181 bytes gzip vs v0.1.1, -89 bytes vs pre-SDK-275. Follow-up SDK-XXX will look at proper scroll tracking on pages with internal scroll containers (capture-phase scroll listening). Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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- README: scroll_depth row no longer mentions the removed aboveFold property or 2s dwell. - autocapture.ts: tighten the setupScrollTracking jsdoc; drop the ticket reference (belongs in the PR description, not source code) and the competitor name-check (changelog territory). - autocapture.test.ts: remove the 'does not include aboveFold property' test — it was a redundant guard once the property was gone from the source, and the existing toEqual assertions cover it. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
nattb8
approved these changes
May 1, 2026
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Summary
Hotfix for SDK-275. The v0.1.1 ResizeObserver fix in #2868 addressed the original "page grew after init" race, but the synthetic
aboveFold: true depth: 100event was still being emitted on SPAs and pages with internal scroll containers. Confirmed on https://godsunchained.com/ wheredocument.documentElement.scrollHeightis permanently equal towindow.innerHeightbecause the page uses a<div class="website-content">withoverflow: autoas its actual scroll container.The bug is architectural — any check based on the document element gives the wrong answer when the document itself isn't the scroller. ResizeObserver doesn't help: the document height is locked, so the "page grew" branch can never trigger.
Fix: drop the synthetic above-fold event entirely. Match PostHog / Segment / GTM behaviour and only fire
scroll_depthon real scroll events. On pages where the document doesn't scroll, no milestone fires. The previous bounce-filter signal on legitimately short pages is lost, but in exchange we eliminate an entire class of false positives — and none of the established lightweight trackers do this anyway, so we're not losing competitive ground.Bundle size
CI will post the authoritative delta. As a sanity check, locally measured: 5,670 B gzip (down 181 B from v0.1.1, down 89 B from the pre-SDK-275 baseline). Removing complexity wins.
Follow-up
A new Linear ticket will be opened to cover proper scroll-depth tracking on pages with internal scroll containers (capture-phase scroll listening, ~+50–100 B), plus documenting the iframe limitation in
docs.immutable.com— that one needs a separate PR againstimmutable/documentation.Test plan
🤖 Generated with Claude Code
Note
Medium Risk
Changes
scroll_depthsemantics by removing the synthetic 100% event on non-scrollable pages, which may affect downstream analytics/dashboards that relied on those events. Logic is localized to autocapture and is covered by updated tests, with no auth/PII impact.Overview
Removes the synthetic above-the-fold
scroll_depthemission (previouslydepth: 100after a dwell viaResizeObserver) and now only emits scroll milestones when the document is actually scrollable and the user scrolls.Updates tests to drop
ResizeObserver/timer-based behavior and assert that non-scrollable pages never emitscroll_depth(including on spurious scroll events), updates README documentation accordingly, and bumps@imtbl/pixelto0.1.2.Reviewed by Cursor Bugbot for commit a327b95. Bugbot is set up for automated code reviews on this repo. Configure here.