LGTM

In addition to the inline comments, I have two generic comments:

• The use of semicolons at the end of the line seems inconsistent. Unless there is a specific reason to keep them, I would just remove them all.
• I would avoid the use ofif (...) return; orif (...) break; or evenif (...) some_functions(). The expression after the condition should go on a new line, and it should be surrounded by{ }, e.g.:

  if(...){   ...}

Fun fact: I originally implemented both the collapsible sidebar (~12 years ago inpython/cpython#47393) and the copybutton (~11 years ago inpython/cpython@59dba38) and since then they have traveled around a number of dirs and repos. It's nice to see that in the meanwhile things got standardized and JS evolved to the point that JQuery is no longer needed.

Working fine now, thanks!