Some window managers (such as notion) treat Pager applications slightly differently from normal clients. The XFCE4 panel is a pager, and can announce itself as such by calling 'wnck_set_client_type' on startup*. This will make it work better on window managers like notion. (not tested, as it seems the latest git depends on garcon-gtk2-1 which does not appear to be available on Debian yet)
Built garcon and made the change, I can confirm it has the desired result. Get it from git at https://github.com/raboof/xfce4-panel
Problem is that wnck start to throw errors the 2nd time the value is set, so with 2 pagers this would be a problem. Now the tasklist and everything else should than technically also set this, but then the errors get even worse.
You're right of course. What would be the proper place to do this? It would make sense to do it early on on startup (after all, the panel *is* a pager). On the other hand, right now only the plugins depend on wnck, the main code does not. Would you be opposed to have a dependency on wnck from the main code? Where should the source indication be set?
Updated patch at https://github.com/raboof/xfce4-panel/tree/source_indication (https://github.com/raboof/xfce4-panel/commit/6776661846ec9d178555500dbbbaf82b33f006ec)
Pushed in d296ca7.
*** Bug 9520 has been marked as a duplicate of this bug. ***
Seems this bug caused regressions, see bug 11055 and its dupe 10747
Fix is correct, closing.