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[Tracker] Use ResizeObserver instead of polling for document height #6174

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[Tracker] Use ResizeObserver instead of polling for document height #6174

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8758ce1
Use ResizeObserver instead of polling for document height
ukutaht Mar 18, 2026
fe96564
fix: avoid forced reflow in getCurrentScrollDepthPx
ukutaht Mar 18, 2026
37381e0
disable engagement tracking in compat mode
ukutaht Mar 18, 2026
1824094
Add changelog entry
ukutaht Mar 18, 2026
0e24d26
Bump tracker script version
ukutaht Mar 18, 2026
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3 changes: 2 additions & 1 deletion CHANGELOG.md
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Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,7 @@ All notable changes to this project will be documented in this file.

- Allow querying `views_per_visit` with a time dimension in Stats API
- Add `bounce_rate` to page-filtered Top Stats even when imports are included, but render a metric warning about imported data not included in `bounce_rate` tooltip.
- Add `time_on_page` to page-filtered Top Stats even when imports are included, unless legacy time on page is in view.
- Add `time_on_page` to page-filtered Top Stats even when imports are included, unless legacy time on page is in view.
- Adds team_id to query debug metadata (saved in system.query_log log_comment column)
- Add "Unknown" option to Countries shield, for when the country code is unrecognized
- Add "Last 24 Hours" to dashboard time range picker and Stats API v2
Expand All @@ -19,6 +19,7 @@ All notable changes to this project will be documented in this file.

- Keybind hints are hidden on smaller screens
- Site index is sortable alphanumerically and by traffic
- Use ResizeObserver instead of polling in tracker for scroll depth. Removes forced reflows caused by the tracker script.

### Fixed

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2 changes: 1 addition & 1 deletion tracker/package.json
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@@ -1,5 +1,5 @@
{
"tracker_script_version": 33,
"tracker_script_version": 34,
"type": "module",
"scripts": {
"lint": "eslint",
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22 changes: 4 additions & 18 deletions tracker/src/engagement.js
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Expand Up @@ -137,10 +137,8 @@ function getDocumentHeight() {
}

function getCurrentScrollDepthPx() {
var body = document.body || {}
var el = document.documentElement || {}
var viewportHeight = window.innerHeight || el.clientHeight || 0
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@ukutaht ukutaht Mar 18, 2026

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By removing IE support for engagement tracking, these fallbacks are not necessary since window.scrollY and window.innerHeight are well-supported:

https://caniuse.com/mdn-api_window_innerheight
https://caniuse.com/mdn-api_window_scrolly

var scrollTop = window.scrollY || el.scrollTop || body.scrollTop || 0
var viewportHeight = window.innerHeight
var scrollTop = window.scrollY

return currentDocumentHeight <= viewportHeight
? currentDocumentHeight
Expand All @@ -151,23 +149,11 @@ export function init() {
currentDocumentHeight = getDocumentHeight()
maxScrollDepthPx = getCurrentScrollDepthPx()

window.addEventListener('load', function () {
new ResizeObserver(function () {
currentDocumentHeight = getDocumentHeight()

// Update the document height again after every 200ms during the
// next 3 seconds. This makes sure dynamically loaded content is
// also accounted for.
var count = 0
var interval = setInterval(function () {
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@ukutaht ukutaht Mar 18, 2026 •
edited
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No need to poll the document for height changes during initial load. Any time DOM is loaded and updated, ResizeObserver will keep currentDocumentHeight variable in sync.

currentDocumentHeight = getDocumentHeight()
if (++count === 15) {
clearInterval(interval)
}
}, 200)
})
}).observe(document.documentElement)

document.addEventListener('scroll', function () {
currentDocumentHeight = getDocumentHeight()
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@ukutaht ukutaht Mar 18, 2026

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No need to recompute document height on every scroll event. If the document height changes at any point, ResizeObserver will keep it updated.

var currentScrollDepthPx = getCurrentScrollDepthPx()

if (currentScrollDepthPx > maxScrollDepthPx) {
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4 changes: 3 additions & 1 deletion tracker/src/plausible.js
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Expand Up @@ -16,7 +16,9 @@ function init(overrides) {

initConfig(options)

initEngagementTracking()
if (!COMPILE_COMPAT) {
initEngagementTracking()
}

if (!COMPILE_MANUAL || (COMPILE_CONFIG && config.autoCapturePageviews)) {
initAutocapture(track)
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