I was hunting down an issue that caused the TOC in HTML5 pages to not show by default. I tracked the issue down to tabpanel1.js. I had to remove anything in gTabListIdArr and gTabContentIdArr that ended in numbers to fix the issue (none of the related pieces have numbers, if this isn't true for you be careful).
Here's my question: How did those extra list items appear in the first place? I can fix this issue manually, by removing them from tabpanel1.js after it's created, but I'd rather know how it happened so I can stop it from happening again.
This issue has now jumped from tabpanels1.js to tabpanels2.js.
The issue will stay jumping because there is probably something in the TOC that the script is breaking on.
This sounds like you are using Multiscreen HTML5. Is that correct? My question: is the TOC not loading? Or is the sidebar being hidden by default?