Improve legibility of element-based navigation when many elements are in close proximity

[mjrusso]

Legibility when using element-based navigation is (usually) OK-ish, but far from perfect.

As an example of a degenerate case, here's what HN looks like when element-based nav is active:

The combination of a small font size, with many unique anchors in close proximity, is problematic. It might make sense to ignore elements that are clustered too closely together (using size as a proxy for importance).

Originally posted by @mjrusso in #1 (comment)

[mjrusso]

I implemented (naive) algorithm that reduce "crowding", by filtering out elements that are positioned too closely together.

Here's what HN looks like with this change (see 323229b for implementation details):

This is a huge improvement over before. It can still be hard to read the text, but that's a separate issue (being tracked in #4).

[mjrusso]

Added 4cd2104, which draws a background colour underneath the element labels, to improve legibility. Here's what HN looks like with this change:

It's not perfect, and not a substitute for #4, but it is a dramatic improvement.