safari intermittently not recording changes in session replay

[pauldambra]

Bug Description

here's what i know so far

this was made more obvious by rrweb-io/rrweb#1597 but i don't think caused by it

• when another library has patched (in the rrweb code "tainted") one of a set of browser functions or objects
• rrweb adds an iframe to the DOM and attempts to load an unpatched version from the window in that iframe
• this is particularly important for angular apps where zone's change detection fights rrweb's and causes performance issues
• this works in chrome
• in safari it doesn't work (or doesn't always work 🤷)
• meaning a recording gets a full snapshot but no mutations

a wonderful customer created https://github.com/rugs65ducat/minimal-repro which lets me recreate this locally

in stepping through what is happening

in both chrome and safari i can load the MutationObserver from the iframe
in both i can init the observer
but if i add e.g. a console log to the callback on observed mutation i only ever see that run in chrome

Debug info

[pauldambra]

if i get mutation observer from window in angular component in safari that does log mutations

[pauldambra]

if i get mutation observer from an iframe in the angular component that also doesn't work

something off between chrome and safari here

[pauldambra]

validated this isn't simply broken in safari and you can otherwise record in safari

[pauldambra]

trying this PostHog/posthog-js#1687

[pauldambra]

safari definitely still hates getting mutation observer from an iframe but we don't do that for Angular zones any more and those were the only reports we had so... 🙈