Enabling the "Wikitext syntax highlighting" beta feature expands the main text box to the full-width of the page, pushing the page scan to the bottom. This makes proofreading works on Wikisource a pain.
EDIT: This seems to be due to CodeMirror making its own text box instead of using prp-page-content if it exists (due to prp-page-content being a part of ProofreadPage).
It seems to work if you only enable codemirror after loading the page for editing. If it's on by default, the bug you see happens. I've opened Phabricator:T176449.
This seems to not be related to CodeMirror, but be a bug in some user scripts. Can you try editing with `&safemode=1` appended to the edit page URL, and see if you still get the problem?
I've found what seems to be the confliction, if you could confirm. "OCR: Enable OCR button in Page: namespace." seems to be the culprit.
After enabling, the problem happens when loading the page with CodeMirror enabled.
jQuery.Deferred exception: fn[command] is undefined cmTextSelection@https://en.wikisource.org/w/load.php?debug=false&lang=en&modules=ext.CodeMirror%2Ccharinsert%2CeventLogging%2CnavigationTiming%2CwikimediaEvents%7Cext.CodeMirror.data%2Clib%7Cext.CodeMirror.mode.mediawiki%7Cext.centralNotice.geoIP%7Cext.centralauth.ForeignApi%7Cext.centralauth.centralautologin.clearcookie%7Cext.echo.api%2Cinit%7Cext.eventLogging.subscriber%7Cext.proofreadpage.page.edit%2Cnavigation%7Cext.proofreadpage.ve.pageTarget.init%7Cext.uls.common%2Ccompactlinks%2Ceventlogger%2Cinit%2Cinterface%2Cpreferences%2Cwebfonts%7Cext.visualEditor.desktopArticleTarget.init%7Cext.visualEditor.supportCheck%2CtargetLoader%2Ctrack%2Cve%7Cext.wikimediaEvents.loggedin%7Cjquery.accessKeyLabel%2CbyteLength%2CbyteLimit%2CcheckboxShiftClick%2Cclient%2Ccookie%2CgetAttrs%2ChighlightText%2CmakeCollapsible%2Cmousewheel%2Cmw-jump%2CprpZoom%2Csuggestions%2CtabIndex%2CtextSelection%2Cthrottle-debounce%7Cjquery.uls.data%7Cmediawiki.ForeignApi%2CRegExp%2CTitle%2CUri%2Capi%2Ccldr%2CconfirmCloseWindow%2Ccookie%2Cexperiments%2Cicon%2CjqueryMsg%2Clanguage%2Cnotify%2CsearchSuggest%2Cstorage%2Ctemplate%2Ctoolbar%2Cuser%2Cutil%7Cmediawiki.ForeignApi.core%7Cmediawiki.action.edit%7Cmediawiki.action.edit.collapsibleFooter%2CeditWarning%7Cmediawiki.api.options%2Cuser%2Cwatch%7Cmediawiki.language.data%2Cinit%7Cmediawiki.libs.pluralruleparser%7Cmediawiki.page.ready%2Cstartup%7Cmediawiki.page.watch.ajax%7Cmediawiki.template.regexp%7Cmediawiki.ui.button%7Cmediawiki.widgets.visibleByteLimit%7Coojs%2Coojs-ui-core%2Coojs-ui-widgets%2Csite%7Coojs-ui.styles.icons-editing-advanced%2Cicons-moderation%2Cicons-movement%7Cschema.UniversalLanguageSelector%7Cskins.vector.js%7Cuser.defaults%7Cwikibase.client.action.edit.collapsibleFooter&skin=vector&version=0d5kzmj undefined load.php:140:200 TypeError: fn[command] is undefined
