Cannot select text inside WebPlatform CSS Inline Docs

[Mark-Simulacrum]

Related to #9440.

I'm guessing that the WebPlatformDocs extension don't stop mousedown events from propagating to prevent the no-focus class from disabling the selection of text.

[peterflynn]

This broke fairly recently -- it was working as of 0.42

[redmunds]

git bisect points to commit marcelgerber@76f0a63 in pull request #8663

[redmunds]

Confirmed. Closing.