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Performance: Lazily syntax highlight blob views

Administrator requested to merge tr/blob-perf into main

Created by: umpox

Note For reviewer: This started pretty experimental, but I think it could be a useful performance UX win. New to this part of the code base so please let me know if I should be doing something differently!

Description

This PR adds an experimental feature that:

  • Fetches and renders formatted but unhighlighted code blobs. We keep the formatting to ensure we don't have a large layout shift when the backend finally returns.
  • Disables the loading spinner on the blob view. I think we can now reasonably say that almost every file (?) should be returned almost immediately, so we shouldn't need this.

Example 1: Compared against the current blob view and syntax highlighting

Noticeable on most files, much less time waiting to view code.

https://user-images.githubusercontent.com/9516420/182154393-b1e31027-16a1-412d-bd4e-11ae12a546c1.mp4

Example 2: Compared against CodeMirror and the newer syntax highlighting

Mainly noticeable on medium/large files, still valuable I think. New syntax highlighting is pretty awesome :)

https://user-images.githubusercontent.com/9516420/182156731-3c24d69b-11a9-410a-abe6-a6f87e6879c3.mp4

Test plan

Tested locally with the flag enabled and disabled

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