Have to set explicit focus to a page by clicking on it, after clicking Back. b=504450 r=josh

widget/src/cocoa/nsChildView.mm

@@ -1039,9 +1039,14 @@ NS_IMETHODIMP nsChildView::SetFocus(PRBool aRaise)
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK_NSRESULT;
 
-  NSWindow* window = [mView window];
-  if (window)
-    [window makeFirstResponder:mView];
+  // Don't so anything if we're invisible (if Show(PR_FALSE) has been
+  // called on us, or if Show(PR_TRUE) hasn't yet been called).  This
+  // resolves bug 504450.
+  if (mView && ![mView isHidden]) {
+    NSWindow* window = [mView window];
+    if (window)
+      [window makeFirstResponder:mView];
+  }
   return NS_OK;
 
   NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;