There is no expand (maximize) button on Shotcut window.
Ubuntu 18 lts, full HD1080 14’’ screen.
Try checking Show Toolbar.
Shotcut does not make the buttons on your main window title bar. On Linux, it is your window manager that is responsible. Shotcut is just using standard Qt window class for this and Qt’s
xcb plugin for X Windows. Maybe there is a problem when View > Show Toolbar is turned off although I did not reproduce it on Ubuntu 18.04 with GNOME Shell. I do not think this is a bug I can address. Take a look at GNOME Tweaks: