Help Viewer has a “Table of Contents” button on its toolbar to show or hide the table of contents. But, the APIs you need to use as a developer to make this button work are secret and undocmented. So, third-party apps have to include their own button and leave the toolbar button greyed out, resulting in something like this rather ridiculous user experience, from the otherwise excellent Affinity Designer:
Help Viewer is misconfigured to put the string “icns” — the filename extension for icon files — onto the icons of help bundles. This is caused by this key in its Info.plist file:
Whoever wrote that seems to think “ICExtension” means “extension for icon files” but it actually means “string to display on icons for files that this app opens”.
So, Help Book icons end up looking like this: