Hoping the stack trace and the context of the fact this only happens on PDFs file pages e.g. https://commons.wikimedia.org/wiki/File:%E5%8F%A2%E6%9B%B8%E9%9B%86%E6%88%90%E6%96%B0%E7%B7%A8059.pdf will help identify the bug in the gadget/deployed code as I'm not too familiar with when PopupWidget is used.
https://logstash.wikimedia.org/app/kibana#/doc/logstash-*/logstash-2020.09.21/clienterror/?id=AXSxTjTzE5thwF5r8Mde
Error message
Error: Permission denied to access property "parentNode"
Stack Trace
at Sizzle</Sizzle.setDocument/contains< URL1:18:486 at Sizzle</Sizzle.contains URL1:20:522 at OO.ui.contains URL1:174:19 at OO.ui.PopupWidget.prototype.onDocumentMouseDown URL1:241:926 URL1: https://commons.wikimedia.org/w/load.php?lang=en&modules=dataValues%2Cjquery%2Coojs%2Coojs-router%2Coojs-ui-core%2Coojs-ui-widgets%2Coojs-ui-windows%2Csite%2Cwikibase%7CdataValues.DataValue%2CTimeValue%2Cvalues%7Cext.centralNotice.choiceData%2Cdisplay%2CgeoIP%2CkvStore%2CstartUp%7Cext.centralauth.ForeignApi%2Ccentralautologin%7Cext.eventLogging%2CnavigationTiming%2CwikimediaEvents%7Cext.uls.common%2Ccompactlinks%2Ci18n%2Cinit%2Cinterface%2Clanguagenames%2Cmediawiki%2Cmessages%2Cpreferences%2Cwebfonts%7Cext.wikimediaEvents.wikibase%7Cjquery.client%2Ccookie%2Ci18n%2Cspinner%2CtextSelection%2Cui%2Culs%7Cjquery.spinner.styles%7Cjquery.uls.data%2Cgrid%7Cmediawiki.ForeignApi%2CString%2CTitle%2CUri%2Capi%2Cbase%2Ccldr%2CconfirmCloseWindow%2Ccookie%2Cexperiments%2Cfilewarning%2CjqueryMsg%2Clanguage%2Cstorage%2Ctemplate%2Cuser%2Cutil%7Cmediawiki.ForeignApi.core%7Cmediawiki.action.edit.editWarning%7Cmediawiki.action.view.metadata%7Cmediawiki.editfont.styles%7Cmediawiki.libs.pluralruleparser%7Cmediawiki.page.image.pagination%7Cmediawiki.page.ready%7Cmediawiki.template.mustache%2Cmustache%2Bdom%7Cmediawiki.ui.button%2Cicon%7Cmmv.bootstrap%2Chead%7Cmmv.bootstrap.autostart%7Cmw.config.values.wbDataTypes%7Coojs-ui-core.icons%2Cstyles%7Coojs-ui-widgets.icons%7Coojs-ui-windows.icons%7Coojs-ui.styles.icons-alerts%2Cicons-editing-core%2Cicons-interactions%2Cicons-location%2Cicons-movement%2Cindicators%7Cpdfhandler.messages%7Cskins.vector.legacy.js%7Cuser.defaults%7Cutil.inherit%7Cwikibase.api.RepoApi%7Cwikibase.datamodel%2Cserialization%7Cwikibase.mediainfo.base%2CfilePageDisplay%2Csearch%2Cstatements%2Culs%7Cwikibase.utilities.ClaimGuidGenerator&skin=vector&version=gb30a
