the left column of the mail app disappeared

You know the left column of the mail app which has inboxes and other mailboxes there is a line which you can adjust the size of, I accidentally dragged the line into the left side until it's gone and I can't figure out how to put left column back I am just missing all the mailboxes. When I put my mouse at the very left side it shows up whenever I hover over it but then it disappears. How do I add it back to the main mail screen? I need it fixed because I use that app very often

Update: I figured it out surprisingly, I dragged the left side of the mailbox that pops up only when my mouse hovers over it into the very left side again and pulled it back out and it looks normal like it was supposed to, but that didn't fix it because it still just retracted whenever my mouse left it, so instead I just opened task manager without closing that window and force quitted the app, after a few tries when I opened it the left window is fixed surprisingly, idk why it does this but there should be a better fix for this because that looked more like a glitch than a actual fix for this problem. I wonder if anyone else had that problem when they pushed the left column into the side of the app. I am not sure if the app now is normal and looks like it supposed as it was before because I never paid attention.