Buttons sometimes overlap code

[azerupi]

In the image above, a long comment on the first line interferes with the button who are rendered on top. This makes the comment hard / impossible to read, but also the buttons less visible. I am not sure yet how we should fix this.

My first idea would be to add a background to the buttons so that even if text is placed underneath it is hidden by a uniform background. However this would only be a partial fix because the code underneath would be impossible to read.

[budziq]

Somewhat related rust-lang-nursery/rust-cookbook#263

I was thinking about making the buttons more prominent. Hmm possibly blink on playpen hover. Maybe I should create a separate issue about buttons not being discoverable.

[ehuss]

I believe this has been fixed since #338.