buffer.lisp: Fix rendering of buffer menu on long pages.

[jmercouris]

Additionally, change the width of the menu to be the same as the content, make it a clearer visual indicator that it operates on the content.

Fixes #3416.

[aadcg]

Thanks for the fix!

A couple of notes for the future:

• Don't forget the changelog entry;
• Always add the issue the PR fixes so that it is automatically closed on PR merge;
• Note that you're using tabs instead of spaces (therefore the additions weren't properly indented);
• Always rebase on top of master.

Fixed all of these and merged.

[jmercouris]

Yes, sorry about that. It has been a while since I've made a proper PR!