setUserWidget was modifying the content of the old event itself,
so when `waitForUserWidget()` checked the content to see if it was
there yet, it was, but because the echo hadn't come back, the
IntegrationManager hadn't rebuilt its list.
In other news, its terrifying that we can just accidentally modify
the content of an event in the store. I'm going to make a js-sdk
PR that freezes the content and see what breaks...
Fixes https://github.com/vector-im/riot-web/issues/10977
d82a05fbae