[wahern]

  After some digging I found that the issue is that whenever
  src is set and then contentDocument is called right after,
  there is a race condition and contentDocument will return
  null because the Document hasn't been created in the script
  thread yet.

  -- https://github.com/servo/servo/pull/14764

I decided not to cherry pick because I figured people could look through themselves and decide. And because culprits are not always found (or at least documented) before a fix is committed. This is how thread races work in any language--they're hard to definitively pin down, not to mention reliably reproduce, but generally easy to fix (or at least make disappear) once something suspicious comes to your attention.

Concurrency is easy when you don't share mutable data. But when you do have to share mutable data--core, performance-sensitive shared data structures--Rust hardly makes doing so "fearless". What Rust does is make it difficult to _accidentally_ share mutable data.