I downloaded your document and viewed it using Chrome. I can confirm that the arrows in the sidebar display of your TOC seem to not work. I then right-clicked on one of the non-functioning arrows, and selected "inspect element" from the popup menu.
This immediately revealed an interesting issue: the right side of the tc-story-river element is overlapping with the left side of the tc-sidebar-scrollable element!
The result of this overlap is that the arrows displayed in the sidebar are effectively "covered up" by the story river.
A quick examination of the $:/ControlPanel Appearance/Theme Tweaks panel shows several relevant settings:
1) The Sidebar Layout is set to "Fixed Story / Variable Sidebar"
2) The Story Right is set to 70%
3) The Story Width is set to 71%
Note that the value of "Story Right" and "Story Width" do not match, and that the Story Width is larger than the Story Right.
If you set the Story Width to 70% (so it matches the Story Right value), it avoids the overlapping of the Story River and Sidebar, and allows the arrows in the Sidebar TOC to work as expected.
enjoy,
-e
Eric Shulman
TiddlyTools.com: "Small Tools for Big Ideas!" (tm) (TWClassic)