You found a bug! We've classified it as
We encourage you to continue the discussion and to find an acceptable workaround while we work on a permanent fix.
It turns out this is a *feature* of the source highlighting tool we are using. It it one we are going to disable in 3.1, but wanted to keep consistency in the 3.0 branch.
Double clicking moves to 'editing' mode, making it easier to select text and copy it out for your own use. Blurring out of the textarea returns the background to the expected style.