Additional chat-related features and issues within Microsoft Teams for business
One of my smart users realized that if she set her zoom in Teams to 115%, the left pane collapses when you drag the drag any side border to narrow the width of the Teams app enough, and when expanded the left pane reappears.
Thank you, I just did this and it worked! As I mentioned previously, I work for a large company and do not have admin access, so I'll take this as a temporary fix. Hopefully Microsoft will make a permanent fix.