I can provide a repro. The issue seems to be this property is being added to the button flex: 0 1 100%; when I change it to flex: 0 1 auto; the issue goes away
relater PR in adaptive cards repo https://github.com/Microsoft/AdaptiveCards/pull/1795/files
Webchat is rendering my signin card like this:

Notes for @corinagum :
ac-pushButton
#bot > div > div > div.wc-message-pane > div.wc-message-groups > div > div:nth-child(4) > div.wc-message.wc-message-from-bot > div > div > div > div > div > div > div:nth-child(3) > div:nth-child(1) > div > button: {
flex-shrink: auto;
}
Thanks Jason. I'll look into this. :)
Seems to be repro of #933
Steps to do:
@JasonSowers I tried to bump to Adaptive Cards 1.1 (in this PR #1231), which I believe they should have this fix (AC 1.1 released just weeks ago, and their fix to this issue released in August).
But unfortunately, looks like they didn't fix it yet (or they didn't fixing our problem). I have commented on their PR and ask them to review it.
So I need them to verify if they really fixed it or not.
We are blocked until the following AC bug is fixed:
https://github.com/Microsoft/AdaptiveCards/issues/1794
Once fixed, we can pick up that version of AC, and push out an updated WebChat
@JasonSowers @corinagum Could you help me to verify one thing?
On your web page that you got this problem, can you check if you have <!DOCTYPE html> specified? If no, can you add it and see if it workaround the problem?
It seems rendering in HTML5 might workaround it.
<!DOCTYPE html> Fixed it 馃憤
馃嵕 馃帀馃檶
Will keep this bug open. I think Adaptive Cards team would love to know/fix this HTML 4 problem.
The tracked bug https://github.com/Microsoft/AdaptiveCards/issues/1794 is still open.