Window resize issue when heading is too long

Another minor issue I noticed is that when I type a heading that is too long (they are nested within other headings btw) the window resize makes the navigation window on the left larger and spill out onto the notes section of the pages.

Example:
in a .md file, I use a very long heading:
```
## Very extremely super duper longish but pretty long heading
```
- Try recreating it and see if it's just me.

I'm using Chrome on a fairly new Macbook Pro 13"

Reproduce the issue in Chrome on macOS using an .md page with nested headings and the long heading shown in the report. Inspect the theme's navigation and notes layout while resizing the window; done means the left navigation remains within its area and no longer spills over the notes section.

Written by the indexing model from the issue text.