Comment 36 for bug 58977

The EWMH provides no way to reserve any area of the screen except the portion at the edges. (Note that in a xinerama setup, there is only one "screen" which spans both monitors, so that rules out reserving the area this bug report talks about) So, the panel can't tell metacity that this area is reserved. metacity-2.14.x does have most of the code necessary to handle reserved area that isn't at screen edges, but not quite all of it. So three bugs:
  1) EWMH needs to be extended to handle xinerama edges too
  2) Metacity needs to support the new hints
  3) gnome-panel needs to use the new hints

EWMH is first, so I'll move to metacity for now and try to remember to bring this up on wm-spec-list at some point.