I tried out reading mode in BDAG when reading something lengthy.
I had several tabs open in the zone. When in reading mode I could see the tabs for all the tabs in the zone.
Clicking on the other tabs revealed empty text panes but I could click between them. Only BDAG showed content because it was the one I entered reading mode on. On Mac the tabs are not present and I just see the one module.
In reading mode in clicking to other tabs I can see the table of contents if I click on the TOC icon. And turning it off works too.
In reading mode if I leave reading mode from one of the these empty tabs instead of from the one I entered it on, the format of the text in the tab that I entered reading mode in will remain in force when I exit reading mode. Going back into reading mode on that tab and exiting it again does not fix it. I have to close and reopen the tab get the non-reading mode format.