Unable to collapse the left side panel in teams anymore

I was having the same issue but was able to resolve it pretty easily! Close out of Teams > Navigate to Settings > Apps > Installed Apps > Locate Microsoft Teams > Advanced Options under the 3-dots > scroll down to Reset and select it. Easier and quicker than doing an uninstall/reinstall.

Thank you this fixed the non-collapsing left sidebar.

And yes, the chats remained there with pinned people and so on. Only thing that was reset was my Teams settings and most noticeably the teams color theme and notification settings. And my second organization account was removed from the list but you could easily add it back. Small price to pay for peace of mind and stopping the hindrance to your workflow.

Previously I had tried reset/repair multiple times with no success. With so many people claiming it works, I tried it again today and it worked!

Right click teams from the start menu. Open settings. Click both repair and reset. Relaunch. It works.

In case it helps, my org's helpdesk advised clearing cache - which worked for me - via these steps:

1. Force close Teams completely > then using the file explorer go here and delete all files and folders in these locations:
2. For NEW Teams
   C:\Users[username]\AppData\Local\Packages\MSTeams_8wekyb3d8bbwe\LocalCache\Microsoft\MSTeams
3. For OLD Teams
   C:\Users[username]\AppData\Roaming\Microsoft\Teams

NOTE this will wipe out any avatars or custom backgrounds, although my settings seem to be intact.

I have this same problem now too, there must have been an update, but I have no diagonal arrows icon, no zoom in my settings or 'collapse app in sidebar'.