site-kit-wp: IntersectionObserver is not defined dashboard error when accessing Analytics tab

Bug Description

There have been reports of the below dashboard error, both occurring from Site Kit 1.31.0 users. This seems to occur only when clicking on the Analytics tab for one user while it only occurs on the primary dashboard tab for another.

One user reported that disconnecting Analytics results in no dashboard notice.

Awaiting further insights.

Can't find variable: IntersectionObserver

    in header
    in DashboardPageSpeed
    in div
    in Layout
    in div
    in LegacyDashboardSpeed
    in FilteredComponent
    in WithFilters(LegacyDashboardModule)
    in div
    in Row
    in div
    in Grid
    in div
    in DashboardApp
    in GoogleSitekitDashboard
    in RestoreSnapshots
    in ErrorHandler
    in Root

https://wordpress.org/support/topic/cant-find-variable-intersectionobserver/ | Open | Awaiting SH info https://wordpress.org/support/topic/site-kit-encountered-an-error-32 | Resolved (by user, unknown cause) | SH info filled in although not complete https://wordpress.org/support/topic/intersectionobserver-is-not-defined/#post-14401649 | Open | awaiting SH info https://wordpress.org/support/topic/site-kit-encountered-an-error-34/ | Open | Awaiting SH info https://wordpress.org/support/topic/fatal-error-3876/ | Open | SH info https://wordpress.org/support/topic/fatal-error-3876/#post-14475607 | Resolved | SH info - Safari specific - MacOS Big Sur, version 11.4.

Additional Context

• SK 1.31.0
• From the impacted URL we have from one reporter only the Yoast plugin is evident, no obvious caching, optimization or other plugins

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Acceptance criteria

• Site Kit should provide a polyfill for the IntersectionObserver, which is loaded in case it isn’t available. See these docs for reference.

Implementation Brief

• Add intersection-observer to our npm dependencies
• Add a new googlesitekit-polyfills.js entry
  • This module will only include feature tests and dynamically require polyfills as needed
  • Checks for intersection observer and load the polyfill asynchronously if the feature is not available
• Add this new entry as a dependency of our vendor asset so that it is automatically loaded with our dependencies
• Merge #3965

Test Coverage

• N/A

Visual Regression Changes

• N/A

QA Brief

Testing Site Kit

• Test intersection observer specific functionality – enable usage tracking, enable PSI and scroll down to view the widget – no error should occur

Browsers

• Test Site Kit using a modern browser (check for errors in the console related to polyfills or intersection observer)
• Test Site Kit using an older browser that does not support intersection observer (e.g. Safari 12.0 or below) – there should be no errors related to intersection observer

Changelog entry

• Fix potential error in older browsers that don’t support IntersectionObserver.