setup a preliminary tag index page with alphabetized tags and counts using built-in hugo functionality (it's easy, can include links to leaf pages by tag as provisional implementation)
trigger button on enter & space bar; transfer focus in, then close button transfers focus back.
disable leaf button when tag is active and leaf does not have tag
make the panel act like a modal (transfer focus; announce; for keyboard user, send them to the close button after tabbing through last tag; return focus to the next leaf after the clicked one)
separate tag from panel: closing panel should no longer close the tag
h2 equivalent label for each branch (possible target from the branch legend in the intro)
NOTE: I am not creating a single page design for each root page, instead I'm proposing all the styles on one page and showing how they would apply to our content (taken from the root data docs on the drive).
the visual styles of the selected tag and the leaves in the course of their interaction – UPDATE: explore other visual styles other than the checkmarks to signify clickable tags.