This indeed seems to be a bug in Teams. However, this is a problematic bug because there doesn’t seem to be a consistent way to reproduce the problem. And without a consistent reproduction any fix will be experimental. Logging is also very limited around this area for privacy reasons.
It would be very helpful if anyone could remember and describe how they got into this state where left and right keys stopped responding.
There are ideas in this thread about GPU acceleration, print screen key, function key, accessibility keys, switching between editing and composing messages etc, but none can consistently reproduce the problem. The search continues.
As a software developer, I know all about how hard it is to triangulate bug reports without a consistent way to reproduce them. I also know no error this serious would be allowed to remain in our products for a year or more. A lot is still known - including a consistently working (if annoying) workaround that at least indicates the type of problem as well as the fact that this error only occurs in the chatbox, and not in other text fields. At some point, you have to set aside time to hunt the bug down yourself and try to reproduce it.
This is the kind of bug no user will *ever* give you a 5 step reproducible report for barring extreme luck. Because we don't use the arrow keys all the time, the bug could have been triggered hours before being noticed.