the more general case of “which tid is focused” in a river is discussed here:
fwiw, I use the Gk0Wk/focused-tiddler plugin for it, where my use-case is solely to style the focused tid in the Open tab (which I have elevated to a SidbarSegment). I got all this from the ‘Seentid’ plugin, as discussed here: Presenting: Seentid - highlight sidebar title seen in viewport