URL: https://app.slack.com/

Browser / Version: Firefox 77.0
Operating System: Linux
Tested Another Browser: Yes Chrome

Problem type: Design is broken
Description: Items not fully visible
Steps to Reproduce:
wide images are clipped in slack threads (with redesigned slack UI)

STR:

1. Create a "thread" in some slack room.
2. Paste this URL (or the URL of some wide image) into that thread: https://pocket-syndicated-images.s3.amazonaws.com/5df8fc2de1fd4.jpg
3. Make your browser window kinda skinny (say, ~1000px wide)

ACTUAL RESULTS: The image runs off the right side of the thread pane.
EXPECTED RESULTS: The image should shrink to fit.

Firefox gives "actual results". Chrome gives "expected results".

This is a version of https://bugzilla.mozilla.org/show_bug.cgi?id=1316534 . I'll post more details below.

View the screenshot

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