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I would like to configure CentOS7 (KDE4) computers with two screen into a special configuration. I have done that before on CentOS5 (KDE3), and now I have to migrate to CentOS7 (and preferably to CentOS8 (KDE5?) when it is available).

I have configured X already to have two screens the way I want them. One is :0.0 and the other is :0.1. On :0.0, I have to run a fullscreen application, which has to have mwm (Motif Window Manager). mwm and the application work fine on screen :0.0.

Now it comes the complicated part. On another screen :0.1, I would like to run KDE, which I will customize. On CentOS5, that was not a problem, since I started kwin, kicker, and kdesktop on that screen by setting DISPLAY environment variable properly. That was enough to have basic KDE functionalities I need there.

Newer KDE is more complex and I cannot start it so simple. Whatever I do, it starts on both screens or does not start at all. I have managed to start kwin. It works, but it writes a lot of error messages on stdout/stderr.

Is there a way to make KDE run on display :0.1 only, and leave :0.0 alone? For me it seems like programmers forgotten about displays, so it seems that some programs within KDE environment are aware of it, while others occupy all displays available. Plasma desktop does seem to ignore every display setting I have tried.

I guess I could run two different window managers on two screens if I started two XServers, but multiseat configuration (https://lxadm.com/Configuring_multiseat_X_workstation) is not the solution I am looking for in this case. Multiseat config then has displays :0 and :1, and brings two completely separate sets of mice, keyboards, cursors on screen. That is not acceptable. I need one keyboard and mouse to go from one screen to another.

The functionality I need on that screen is to have a customized taskbar, where users can start only allowed applications available there. They shall not have access to other apps. This can easily be accomplished with KDE, but now I have problems with running it on one screen only.

I am not sure I will be able to pull this off with KDE, which is available in CentOS7. Maybe I should try it with GNOME, which is not so highly customizable as KDE. For the first test it started only on one display and did not mess with :0.0. However, I am not sure I can limit its use as I can in KDE.

Is there some other desktop environment, which would provide simple configurable taskbar and would not mess with the window manager on another screen? I am thinking about trying xfce.

  • I don't think that you can do that reliably. Window managers will want to have input devices all to themselves (eg. to switch with Alt-Tab), forcibly set the focus, etc. They won't accept to share the Xserver with another wm, even if running on another screen. (In fact they could, but they will have to be re-designed from scratch & completely against the rigid maximalist philosophy of gnome or kde). Having two Xservers collaboratively share the input devices feels a lot more feasible, though I have no idea how to do that exactly ;-) – mosvy Jun 8 at 10:20
  • Anyways, notice that any client connected to an X11 display has access to the whole display, ie can remove the windows of other clients, fetch data from them, just if they were its windows, and no matter if they're on the same screen or not. There's no enforced separation. If you think that the user will be only able to use the allowed applications, think again. – mosvy Jun 8 at 10:25
  • We have been using such configuration for years. Running different window managers on different works. Screens are configured in a way that windows cannot be moved from one screen to another, while keyboard and mouse are shared between them. It is not a problem if user can run any application. The intent is to hide all apps from main menu to make environment simple with limited and strictly defined functionalities with no distraction elements. It is actually desirable for administrator to start any app via Alt+F2. I am talking about highly controllable environment with responsible users. – nobody Jun 10 at 6:27
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The solution I have found is to use xfce4 instead of KDE. I can start mwm on first screen. After that I start xfce4 window manager xfwm4 with --display=:0.1 option. For my needs it is enough to start also xfce4-panel and xfdesktop. This provides enough functionalities to use second screen for running applications that can be started with launchers from xfce4-panel. xfce4-panel can be configured in kiosk mode where users cannot change anything. On first screen fullscreen app with motif window manager also works fine.

If I start xfwm4 first, it takes over both screens and then mwm cannot be started. If mwm is started first on primary screen, xfwm4 works fine on the second screen as it should.

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