VoiceOver will only read a11y elements that are currently on screen, so if the toolbars are scrolled out of view due to our fullscreen implementation, it's possible that VoiceOVer users can be stuck in a situation where they are unable to access the navigation controls in the secondary toolbar.  FullscreenModel does not update toolbar visibility for programatic scroll events, so it's not possible to hide the toolbars for VoiceOver scrolling.  However, it's technically possible to get into this situation if the user scrolls with voiceover disabled then enters voiceover mode while the toolbars are hidden.