I have a bookdown bs_4 book with a long main (left-hand) toc with many parts, each with many chapters. So if I am reading chapter 99, although the chapter heading is highlighted in the toc sidebar, and even though the toc does have its own scrollbar, the heading usually isn't visible because it is below the bottom of the screen. Is there a way to either

a) collapse the toc to Part level so the toc lists only Parts and expands only chapters for the currently active Part or

b) use some js to always scroll the toc to the active heading?