The goal is to have some tests for the snap package of Firefox and they're using Selenium.
The test takes an image from the canvas and then compare it to a reference: the problem is to know when the canvas is ready.
So the idea is to set a listener for pagerendered but afaik there is no guarantee that the listener will be set before the event is triggered, hence this patch.
That said, if you've a better/simpler idea to achieve the same, please share it.

The goal is to have some tests for the snap package of Firefox and they're using Selenium.

How is the PDF viewer being loaded in this case, because it might be possible to work-around that?

Anyway, could we at least avoid adding a new Preference for this and keep it as "just" an option?
If we change this event to be dispatched in all builds, could the Selenium-tests listen for "webviewerloaded"[1] and then call e.g. window.PDFViewerApplicationOptions.set("hasInitializationHook", true) from that event listener instead?

Edit: Another possibility to avoid the Preference, assuming that isInAutomation === true when the Selenium-tests run, might be to just use the "initializationhook" hash-parameter there as well.

[1] Since that event is dispatched before the viewer-initialization runs.

Closing in favor of #18617 which only requires changing test code and overall seems safer/simpler.