Thank you for the quick fix! Unfortunately it didn’t completely work:
Something I failed to call out the first time is that I’m using the dropdown top sidebar instead of the left aligned sidebar. I don’t know if that makes a difference. But the top of the chat is now completely cut off:
The update did fix the admin panel sidebar though, which I didn’t know wasn’t supposed to scroll like that! 
So that’s good.
Also it does say that the composer heights are not defined variables:
ETA: I played around with it and it is definitely a bug with the header dropdown menu option 
And it remains broken if you do the header dropdown menu and the separate chat nav:
