Tree page when at 400% zoom only lets you see a couple notebooks at a time when scrolling due to top 2/3rds of page being fixed on the page.

**Issue Area # 2**
- **Expectation**: Can the page be zoomed to 400% zoom in a 1280px wide display without requiring two scroll bars? (WCAG Criteria 1.4.10 (AA)).
- **Observed**:
- Minor annoyance on /tree page. The top 2/3rds of the page is fixed so you can only see 1-2 notebooks from list as you scroll. Recommend making most (everything below the main header) if not all of it static to maximize scrollable area.

Start at the /tree page and reproduce the layout at 400% zoom on a 1280px-wide display. Inspect the fixed regions and scrolling behavior below the main header; done means the page exposes substantially more notebook entries without requiring two scroll bars and still satisfies the stated WCAG criterion.

Written by the indexing model from the issue text.