On GTK, middle-click on the title bar (and the window frame in general) will...

chrome/browser/gtk/browser_window_gtk.cc

Index: chrome/browser/gtk/browser_window_gtk.cc
===================================================================
--- chrome/browser/gtk/browser_window_gtk.cc	(revision 22722)
+++ chrome/browser/gtk/browser_window_gtk.cc	(working copy)
@@ -1549,8 +1549,8 @@
     return TRUE;
   }
 
-  // Handle left and right clicks.  In particular, we care about clicks in the
-  // custom frame border and clicks in the titlebar.
+  // Handle left, middle and right clicks.  In particular, we care about clicks
+  // in the custom frame border and clicks in the titlebar.
   GdkWindowEdge edge;
   bool has_hit_edge = browser->GetWindowEdge(static_cast<int>(event->x),
       static_cast<int>(event->y), &edge);
@@ -1587,6 +1587,9 @@
         return TRUE;
       }
     }
+  } else if (event->button == 2) {
+    gdk_window_lower(GTK_WIDGET(browser->window_)->window);

[CalebEgg_gmail.com, 2009/08/07 09:09:38]

Won't this need to be wrapped in if (has_hit_titlebar) {} like the above and
below conditions?

+    return TRUE;
   } else if (event->button == 3) {
     if (has_hit_titlebar) {
       browser->titlebar_->ShowContextMenu();