menushell: don't activate a menu entry when opening a big menu

If a menu is opened and it doesn't fit entirely below or above
the menu bar, gtk+ will place it on top. The button release will
then activate the popup item that happens to appear under the
cursor. Avoid this by ignoring release events if they originated
in the parent menu bar and the duration of the press was too short.

https://bugzilla.gnome.org/show_bug.cgi?id=703069

gtk/gtkmenushell.c

@@ -809,6 +809,18 @@ gtk_menu_shell_button_release (GtkWidget      *widget,
   GtkMenuShell *menu_shell = GTK_MENU_SHELL (widget);
   GtkMenuShellPrivate *priv = menu_shell->priv;
 
+  if (priv->parent_menu_shell &&
+      (event->time - GTK_MENU_SHELL (priv->parent_menu_shell)->priv->activate_time) < MENU_SHELL_TIMEOUT)
+    {
+      /* The button-press originated in the parent menu bar and we are
+       * a pop-up menu. It was a quick press-and-release so we don't want
+       * to activate an item but we leave the popup in place instead.
+       * https://bugzilla.gnome.org/show_bug.cgi?id=703069
+       */
+      GTK_MENU_SHELL (priv->parent_menu_shell)->priv->activate_time = 0;
+      return TRUE;
+    }
+
   if (priv->active)
     {
       GtkWidget *menu_item;