The first CSS (div.editor-toolbar div button) selector I think only works within the markdown editor, the one I just posted works in the richtext editor as it has a different toolbar. The pic you posted before has a joplin-tinymce div which must mean you had the richtext editor on.
It also explains why clicking it when you saw it made it vanish, it doesn't exist on the editor you swapped to - the CSS was removing it.
