About
Stack Overflow's Challenges can quickly become overwhelming when trying to track which posts you've already read or navigate back to someone's post. There are no pagination and posts are usually quite long, making scrolling at the very best tedious (if you're lucky enough to not scroll past what you need). This userscript provides a relatively clean navigation experience by adding view management and organization tools.
The script adds four main features:
Navigation menu: A simple dropdown menu showing all posts with author names, dates, and preview text, ordered based on the order that they appear on the page with a blue indicator (if you have moderation privileges, you'll see the deleted posts at the bottom with a red indicator).
Individual post management: Each posts gets a "Viewed:" checkbox that you can check when finished reading. Checked posts automatically collapse to show just a compact banner with author name and date information. You can hit "Show" to view the content and collapse it again by hitting "Hide". Viewed posts get a green indicator in the dropdown menu.
Global toggle: An "Expand/Collapse All Viewed" button that lets you quickly show/hide all posts that you've marked as viewed.
Collapsed deleted posts for moderators. All the deleted posts are collapsed by default. You can expand them individually as needed.
Your viewing preferences persist across page reloads, so you can return to a Challenge and immediately see which posts are new since your last visit.

Installation
Click to install with a script manager. N.B. I have only tested this with Tampermonkey on Firefox.
Repository
Here is the source code for this userscript; source code repo
Special thanks to VLAZ for their code review and suggestions.