With XMonad it's also pretty straightforward. You can do it for specific applications by using the window class in your config's manageHook (like: className =?

With XMonad it's also pretty straightforward. You can do it for specific applications by using the window class in your config's manageHook (like: className =? "KeePassXC" --> doFloat). In my configuration, I also have it set up that if I hold the mod key and drag a window with the left mouse button, it becomes floating. Mod+drag with the right mouse button resizes it, pretty similar to the i3 setup you mention.