Changing comment, since I figured out why we're doing things in a wacky way.

No bug.

xpfe/appshell/src/nsContentTreeOwner.cpp

@@ -124,8 +124,14 @@ NS_INTERFACE_MAP_BEGIN(nsContentTreeOwner)
    NS_INTERFACE_MAP_ENTRY(nsIWebBrowserChrome2)
    NS_INTERFACE_MAP_ENTRY(nsIInterfaceRequestor)
    NS_INTERFACE_MAP_ENTRY(nsIWindowProvider)
-   // XXXbz why not just implement those interfaces directly on this object?
-   // Should file a followup and fix.
+   // NOTE: This is using aggregation because there are some properties and
+   // method on nsIBaeWindow (which we implement) and on nsIEmbeddingSiteWindow
+   // (which we also implement) that have the same name.  And it just so
+   // happens that we want different behavior for these methods and properties
+   // depending on the interface through which they're called (SetFocus() is a
+   // good example here).  If it were not for that, we could ditch the
+   // aggregation and just deal with not being able to use NS_DECL_* macros for
+   // this stuff....
    NS_INTERFACE_MAP_ENTRY_AGGREGATED(nsIEmbeddingSiteWindow, mSiteWindow2)
    NS_INTERFACE_MAP_ENTRY_AGGREGATED(nsIEmbeddingSiteWindow2, mSiteWindow2)
 NS_INTERFACE_MAP_END