Vertical outline on smartphone

I mostly use Audible on my smartphone (Android) and there the section outline is listed horizontally above the article text. This is very inconvenient: I can usually see only one heading at a time and then horizontal scrolling is less natural than vertical scrolling. Please display the outline vertically on the smartphone as it is on the computer (or at least make this an option that the user can choose). The vertical outline would probably need to be collapsible so that we could click a button to either display or hide the full outline.