I think i got now a Solution for your needs (without compiz) :
1. Create a keboard-shortcut with the command :
and append it to Alt+W ....... (or whatever you want)
2. open up xclock
Do a right mouseclick on window-title and choose "always on top"
3. open up firefox in fullsceen mode.
Now xclock will disappear.
Use the keyboard-shortcut (see above)
In the upcoming window choose xclock.
Now xclock shows up on top of the firefox fullscreen-window.
And it will stay there ..... even if you scroll- or do other things in that screen
Tested with firefox in fullscreen-mode and waterfox in fullscreen-mode
See vid showing ...... it works !
For those who are interested to try it out :
xclock should be already installed on your system - so you dont need to install it.
For start of xclock i created also a keyboard-shortcut with command :
and bound it to ALT+C
edit 2 :
Even if there exists now a solution for xclock, i noticed, that this solution is not applicable to other apps that are waiting for user input in graphical mode.
Dont try to do it with vlc, xplayer, libreoffice e.g ...... it will not work !
So let me refine my answer, i gave in a post before :
In general it is impossible, to achieve it by using the "normal" preferences, but it seems, that there are exceptions on setting focus on windows.
(may be under use of compiz possible ? - i dont know ! ...and will not try it)
It is actually logical because the browser itself is waiting for user input.
So if I open an app that stays on top of the browser and is waiting for user input, and the
browser (in fullscreen-mode) is also waiting for user input, a click into the browser window will set the focus to its window, and move the application, that is on top of it into the background of the workspace .
One of the exceptions is xclock, which is based on x11-apps, while it runs in its own window, and doesnt wait for user input.
I will have a look at its sourcecode to get more information about the fact, that it works on top of browser, which is in fullscreen-mode, by use of xfdesktop-command.