The sidebar that shows your teams and chats in Microsoft Teams can be hidden when you don’t need it. Microsoft has made changing this setting very unintuitive, but it’s easy once you know where it’s hiding.
Update, 2/11/22: Unfortunately, Microsoft has changed this in modern versions of Teams. It no longer appears to be possible to hide the sidebar if it’s on. The sidebar will automatically hide when you open certain panes (like Calendar), but it’s not possible to hide it in other panes (like Chat.)
Having a sidebar or navigation pane on the left side of an app is perfectly normal—almost every email client has one, for example—and many apps give you the option to toggle it on or off. Microsoft Teams is no exception, but for some reason, the company made the toggle hard to find, and you can’t toggle the sidebar on and off if you have a chat open.
This basically means that you can toggle the sidebar on and off when you have a tab open in a channel, as long as it’s not the “Posts” channel. You can’t toggle the sidebar on and off if you’re in a chat.
To toggle the sidebar, open a tab in a Teams channel and click the two diagonal arrows found to the right of the tabs.
This will hide the sidebar. To show it again, click the diagonal arrows again.
A clue to Microsoft’s thinking is that if you hover over the diagonal arrows, the tooltip reads “Expand Tab” if the sidebar is visible, or “Collapse Tab” if the sidebar isn’t visible. Clearly, the design team was thinking more about giving extra screen space for the tab you’re looking at, rather than letting you hide the toolbar just because you don’t want to see it.