URL: https://web.whatsapp.com

Browser / Version: Firefox 55.0.3
Operating System: Windows 10
Tested Another Browser: Yes

Problem type: Site is not usable
Description: Serious slow down while using it zoomed in
Steps to Reproduce:
I'm well aware of the fact that there's a bug opened[1] on Bugzilla regarding performance problems with WhatsApp Web, but...

What I want to report here is different (or so I think): while using the built-in zoom of Firefox, WhatsApp Web slow down to a crawl rightly after opening and closing some images or videos.

The steps to reproduce are:

1. Open WhatsApp Web (https://web.whatsapp.com/)
2. Set a zoom to the web page (like 110% or 120%, for example)
3. Log in
4. Enter in a chat and try to open and close some images or videos

Expected result:

Right after closing a image/video, WhatsApp Web UI slow down to a crawl. If you try to interact with the UI, it's like everything is running at 1 frames per second.

However, if I reset the zoom level as soon as this happens, the slow down vanishes instantly. Likewise, if you use WhatsApp Web without zoom, the problem doesn't happen at all.

Can you guys reproduce it? If so, does this demand a new bug ticket? I don't know if this is related to the bug mentioned above or not.

[1] https://bugzilla.mozilla.org/show_bug.cgi?id=1376536

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