Browser tab hangs when verifying pantalaimon session

### Description

When verifying a pantalaimon session, getting to the end of the emoji verification flow causes Element Web to hang with high CPU usage. When using Firefox, the "tab is slowing down your browser" prompt appears.

No such hang occurs when verifying from SchildiChat (and likely Element Android).

This happens to me for both my matrix.org account and a self-hosted account.

### Steps to reproduce

- Log into your account with Element Web
- Log into the same account with pantalaimon
- Follow Element Web's prompt to verify the pan session (might need to follow the steps in https://github.com/matrix-org/pantalaimon/issues/71)

After verifying that emojis match, the browser tab may hang, either before or after the "Verification successful" graphic with the green shield.

### Version information

- **Platform**: web

For the web app:

- **Browser**: Firefox 81
- **OS**: Fedora 32
- **URL**: app.element.io / self-hosted version of Element Web (both on v1.7.8)