I think there is something to be said for having some kind of visual element whose job it is to track "where the reader is in the text".
Lacking this, I (and I suspect many others) improvise by using a mouse selection, and constantly growing the selection by shift-clicking. This breaks if the UI has messed with the relevant UI events (e.g. disabling selection or popping up something on mouse clicks).
I wouldn't mind some controls for this designed specifically around the activity of reading. Maybe tap to advance one sentence or one paragraph. Keep the current paragraph entirely in view (unless it's too big; then keep the current sentence entirely in view). Shift the text a paragraph at a time. Make it advance both the marker and the text, so the reader doesn't have to manage those two things separately.
I'd especially like a good tablet solution to this, since reading is my primary activity on a tablet, and there's no mouse in that case.
Although as far as I can tell, Circa Solar isn't open source. But maybe there's another similar thing that is?
I always liked the idea of an "apparent solar time" watch face, and never did find a way to do that with Apple Watch.