Hi, I've got Kdenlive 17.12.3 (it's the only version that I could get working) on Linux Mint 19.3. A while ago, I accidentally checked an option to always open the editor in fullscreen. I unchecked it and clicked the option to leave fullscreen, but nothing happened.
It's getting frustrating. Could someone help me figure out what I did wrong?

Kdenlive have several fixes on last year.. and more other on 5 years i think that you need to switch to a more updated version of Kdenlive and i can understand that the issue is with your old OS... i'm also suggesting to switch to a more updated version of Mint.

BTW.. just delete kdenliverc file to restore kdenlive to default serttings ( https://community.kde.org/Kdenlive/Configuration )