x11-display: add support for _NET_WORKAREAS_Dn

In addition to existing property set also new _NET_WORKAREAS_Dn property
where n is desktop number (between 0 and _NET_NUMBER_OF_DESKTOPS - 1).

https://mail.gnome.org/archives/wm-spec-list/2018-December/msg00000.html

src/x11/atomnames.h

@@ -141,6 +141,7 @@ item(_NET_WM_STATE_FULLSCREEN)
 item(_NET_WM_PING)
 item(_NET_WM_PID)
 item(_NET_WORKAREA)
+item(_NET_WORKAREAS)
 item(_NET_SHOWING_DESKTOP)
 item(_NET_DESKTOP_LAYOUT)
 item(_NET_DESKTOP_NAMES)

src/x11/meta-x11-display.c

@@ -904,6 +904,57 @@ set_workspace_names (MetaX11Display *x11_display)
   g_string_free (flattened, TRUE);
 }
 
+static void
+set_workspace_work_area_hint (MetaWorkspace  *workspace,
+                              MetaX11Display *x11_display)
+{
+  MetaMonitorManager *monitor_manager;
+  GList *logical_monitors;
+  GList *l;
+  int num_monitors;
+  unsigned long *data;
+  unsigned long *tmp;
+  gchar *workarea_name;
+  Atom workarea_atom;
+
+  monitor_manager = meta_backend_get_monitor_manager (meta_get_backend ());
+  logical_monitors = meta_monitor_manager_get_logical_monitors (monitor_manager);
+  num_monitors = meta_monitor_manager_get_num_logical_monitors (monitor_manager);
+
+  data = g_new (unsigned long, num_monitors * 4);
+  tmp = data;
+
+  for (l = logical_monitors; l; l = l->next)
+    {
+      MetaRectangle area;
+
+      meta_workspace_get_work_area_for_logical_monitor (workspace, l->data, &area);
+
+      tmp[0] = area.x;
+      tmp[1] = area.y;
+      tmp[2] = area.width;
+      tmp[3] = area.height;
+
+      tmp += 4;
+    }
+
+  workarea_name = g_strdup_printf ("_NET_WORKAREAS_D%d",
+                                   meta_workspace_index (workspace));
+
+  workarea_atom = XInternAtom (x11_display->xdisplay, workarea_name, False);
+  g_free (workarea_name);
+
+  meta_x11_error_trap_push (x11_display);
+  XChangeProperty (x11_display->xdisplay,
+                   x11_display->xroot,
+                   workarea_atom,
+                   XA_CARDINAL, 32, PropModeReplace,
+                   (guchar*) data, num_monitors * 4);
+  meta_x11_error_trap_pop (x11_display);
+
+  g_free (data);
+}
+
 static void
 set_work_area_hint (MetaDisplay    *display,
                     MetaX11Display *x11_display)
@@ -923,6 +974,8 @@ set_work_area_hint (MetaDisplay    *display,
       MetaWorkspace *workspace = l->data;
 
       meta_workspace_get_work_area_all_monitors (workspace, &area);
+      set_workspace_work_area_hint (workspace, x11_display);
+
       tmp[0] = area.x;
       tmp[1] = area.y;
       tmp[2] = area.width;