libAdwaita is for GNOME only
No matter what happens upstream, we will always do our best to make each Linux Mint release a better experience than the one you already have. Applications will be native and look native. They will integrate well. If we let you choose a desktop theme, ALL installed applications will support it.
If an application doesn’t support Cinnamon we can’t ship with it in our Cinnamon edition. The same goes for MATE and Xfce.
It would be completely unacceptable for us to ship with an application which used its own window controls and didn’t follow the system theme. Looking at it long-term, we also do not want our apps to be designed by people who have no consideration for what is important to us, and whose decisions are motivated by a desktop we don’t even use.
This is File Roller 3.42. This application has always been labeled as “for GNOME”, but it integrated well in any GTK desktop. With File Roller 44 this is no longer the case. It looks just like GNOME Scan in the previous screenshot. It’s not made for MATE, Cinnamon or Xfce and it really shows.
By moving to GTK4/libAdwaita this app really became a GNOME app, an app which looks specifically designed for GNOME and nothing else.