There is a problem with DaVinci Resolve 15.1.2 in Linux version, that it is not showing control buttons as well as title bar for main application window. In this video Ill show you a crutch, that workarounds it.
Importable .kwinrule file may be downloaded from here: gist.github.com/Ashark/74034a4fa658e8af13bf91acc7b4ce2e#file-davinci-resolve-main-window-always-with-titlebar-and-frame-kwinrule