rustdoc: avoid whole page layout on each frame

This makes two changes, based on experimenting with different browsers:

- It debounces resizing the body text. This improves behavior on huge
  pages like struct.Vec.html, because it doesn't have to do layout.

- It does the sidebar width updates directly on the sidebar instead of
  doing it on the `<HTML>` element. Doing it on `<HTML>` causes it
  to recalculate CSS for the entire document, also causing layout jank.

Michael Howell committed 2y ago

77fa09d9a1ddc7f99b0cda10c26688fe6f2b5519

Parent: 9aabfd5