The CodeMirror icon has a border on its right-hand side
This is because it is in a new toolbar group all on its own. WikiEditor thinks the 'tools' group is the last one, and removes its border (so that none appears before the arrow for 'Advanced'):
.wikiEditor-ui-toolbar .group-insert { border-right: 0; }
Maybe this is something WikiEditor should handle? It does after all decide where the new group gets inserted and so could change how it handles the border-removing.