Closes #ISSUE
From notes:
```markdown
- [x] Clicking on the disclsoure icon button in the root-level tree view item should steal focus and move it to the root item (not the icon button)
- [x] [@ben] Allow left/right arrow keys to expand/collapse root tree view items in the nav
- [x] With this, make enter/space work the same as clicking (activate page, don't expand root items, focus moves to the content and leaves nav — becomes consistent with mouse interaction)
- [x] Smart cmd-shift-e: toggling focus should take you to the selected item
- [x] [@ben] pageup + pagedown in nav -> jump between root items
- [x] [@ben] home + end buttons should work
- in nav:
- home always goes to first section header
- end always goes to last _visible_ item (does not expand)
```
Release Notes:
- N/A *or* Added/Fixed/Improved ...
3d200a5466