All good, just a few issues, however a bit annoying:
1. No option for font size in tree view (hardcoded 11pt, why?)
2. A 3-panel layout (like as in NewsFox) is highly appreciated. Yes, you have to change several css rules from flex to block and place one of drawers horizontally. Sorry if it breaks your conception of modern design.
3. Definitely need an option for hotkeys customization.
4. Not an ideal UX, e.g. there is no way to switch from full article to headers list or tree view without mouse.