On Linux I believe that what happens to the keyboard focus depends on the window manager in use and how that window manager is configured to behave. For instance, some window managers have a 'focus follows mouse' setting, but I always hated that so I would disable it, meaning I'd have to click in a window for it to acquire focus. (Or, in my case, use the keyboard to select the window to focus). I believe many window managers also allow you to control what happens to keyboard focus when a new window is opened and you aren't using 'focus follows mouse', but it's been a while since I've played with a window manager that uses floating windows, so I could be misremembering.