Xfce: Tab Key No Longer Navigates Within Some Dialog Windows

Xfce 4.10; Gtk 2.24.25 For example, zenity --question invokes an austere zenity question dialog window with default verbiage and two buttons, as expected. But, with focus on said window, pressing the **Tab** key does not visually appear to rotary the internal focus (as evidenced by a faint, dashed rectangle) through either of the two buttons. This just started happening recently and I've been unable to find a solution - any ideas for smarter searches appreciated!