Hi, I am running ubuntu 11.04 and xfce 4.8 on 2 display machine and sometimes panel is missing all windows from specific display. I configured 2 panels on each display with parameter to display only windows from associated display. If you need more info I will gladly provide it.
I have the same setup on Arch linux; 2 displays, one panel on each display, window buttons in each configured to show only windows for that monitor. What I'm seeing is that all windows show up correctly initially, but after a while some of the buttons go missing. I have been able to pinpoint which action actually triggers the window buttons to disappear. I only notice this when it's too late already.
I have noticed when using Matlab, a Java-based GUI. I can reproduce it reliably: Start Matlab, select Help -> Product Help, then back in the main window, File -> New -> Class. Then close the Editor while watching the Panel. Both the Editor and main MATLAB window buttons will disappear, even though Matlab is still running. I think this is the same issue with Java-based GUIs that affected the gnome panel, see: https://bugs.launchpad.net/ubuntu/+source/libwnck/+bug/144406 and http://cgit.compiz.org/compiz/plugins/workarounds/commit/?h=compiz-0.8&id=79f11ed2cd71d2155a5e04aa8803c56633b212bb. From that page: "Certain java applications set themselves to be transient when they open a modal dialog, causing them to disappear from the taskbar window list. This workaround notices when the transient property is unset and adds the window back to the taskbar." I've taken a peek at the source for the tasklist but I'm not sure how to work this in.
I'm only seeing this with IntelliJ IDEA; also a Java-based application. I was talking to a friend yesterday about this and he sees the same behaviour with Java apps.
I had a more in depth look at the ubuntu bug that was linked to. The workaround for Java windows was done in compiz. If you extrapolate that to xfce then the same fix should probably be done in xfwm rather than in the window buttons panel plugin.
More data: Using gnome-panel with metacity, the bug DOES NOT occur. Using xfce4-panel with metacity, the bug DOES NOT occur. Using gnome-panel with xfwm4, the bug DOES occur. Using xfce4-panel with xfwm4, the bug DOES occur. So, that would seem to point to the window manager rather than the panel.
maybe this would help to fix it: http://youtrack.jetbrains.com/issue/IDEA-17899#comment=27-169762 (look for explanation by Serge Baranov)
also note, that this is not happening only for 2 display machines as stated earlier
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