I had the same issue on Kubuntu 18.04.4, but I was able to overcome it by disabling the window style under appearance (Disable CSD). See attached screenshot.
Hope this helps! ** Attachment added: "Screenshot_20200302_144653.png" https://bugs.launchpad.net/ubuntu/+source/breeze-gtk/+bug/1758756/+attachment/5332785/+files/Screenshot_20200302_144653.png -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1758756 Title: Can't resize GTK 3 windows by clicking in the border To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/breeze-gtk/+bug/1758756/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
