This is consistent with a bunch of other viewer code, since an `AbortSignal` can only be aborted once any "abort" listeners can thus be removed when invoked.
It's width was a bit wrong because of its box-sizing property and it was causing some issues when resizing it with the keyboard.
And for the thumbnails sidebar, the tabindex was missing and some aria properties too.
For now its use is limited to the comment sidebar but it'll be used for the new one
containing the thumbnails.
And make the sidebar more accessible with the keyboard or a screen reader.