This fixes a lot of "jumpiness" and removes the `new_line_pos` stuff.
Closes#2260.
r? @mbrubeck
cc @metajack
Source-Repo: https://github.com/servo/servo
Source-Revision: d3478954fd8e3a8374104984efb91a918b5dbb7e
On my laptop, running `./mach test-unit` goes from about 11 minutes to 22 seconds, when run after `./mach build`.
Fix#5291.
Source-Repo: https://github.com/servo/servo
Source-Revision: 017d1053617f6769d7f355e4a97d52ae67e53d1c