In the Responsive HTML5 layout settings (RH 2015), go to "Basic settings" and set the default pane to "filter".
With this setting, there is no way for the user to access either TOC, Index or Glossary in mobile view ("Phone"). The "sandwich" icon (yes, this one http://bit.ly/1S3q9If) in the upper left opens the filter pane (= the default pane). Next to the sandwich icon, there's the search icon and (again) the filter icon, but no TOC/Idx/Glo icons.
Please fix this. I think the easiest solution would be to implement two options: Default pane (Desktop) and Default pane (Mobile). One sets the default pane for desktop display, the other one sets the behaviour of the "sandwich" icon.