There is code in CodeMirrorHooks::onResourceLoaderRegisterModules() that loads the ext.wikiEditor module from the WikiEditor extension if it is available. This module is only actually needed if the user has enabled the "enhanced editing toolbar" in their prefs (i.e. the "usebetatoolbar" user option). If they aren't using the enhanced toolbar, it shouldn't load the module, otherwise you get weird problems like a double border on the editing area.
Unfortunately, you can't successfully call $wgUser->getOption() from CodeMirrorHooks::onResourceLoaderRegisterModules(). You also can't call WikiEditorHooks::isEnabled( 'toolbar' ) since it uses $wgUser->getOption(). A more clever solution is needed.