Stack Trace
jquery.webfonts.js?fb6f4:65 Uncaught TypeError: (language || this.language || "").toLowerCase is not a function
at WebFonts.getFont (jquery.webfonts.js?fb6f4:65)
at HTMLFormElement.<anonymous> (jquery.webfonts.js?fb6f4:217)
at Function.each (jquery.js?11c05:367)
at jQuery.fn.init.each (jquery.js?11c05:202)
at WebFonts.parse (jquery.webfonts.js?fb6f4:201)
at WebFonts.init (jquery.webfonts.js?fb6f4:82)
at new WebFonts (jquery.webfonts.js?fb6f4:31)
at HTMLBodyElement.<anonymous> (jquery.webfonts.js?fb6f4:478)
at Function.each (jquery.js?11c05:367)
at jQuery.fn.init.each (jquery.js?11c05:202)
ext.uls.displaysettings.js?a4cd3:433 Uncaught TypeError: Cannot read property 'list' of undefined
at DisplaySettings.prepareFontSelector (ext.uls.displaysettings.js?a4cd3:433)
at DisplaySettings.prepareContentFonts (ext.uls.displaysettings.js?a4cd3:522)
at ext.uls.displaysettings.js?a4cd3:558Impact
Webfonts are on one particular page.
Notes
This happens because we access the lang attribute as element.lang. Normally this returns the attribute value, but for form elements, it returns the element with name="lang" instead. This is an element object (so truthy) which does not have toLowerCase method.
<form id="mw-newentity-form1" action="/wiki/Special:NewItem" method="post" enctype="application/x-www-form-urlencoded" class="mw-htmlform mw-htmlform-ooui oo-ui-layout oo-ui-formLayout"> [...] <input type="text" tabindex="0" aria-disabled="false" name="lang" value="fi" id="ooui-php-2" class="oo-ui-inputWidget-input webfonts-changed" autocomplete="off" aria-expanded="false" role="combobox" aria-owns="ooui-1" aria-autocomplete="list" /> [...] </form>
Not that there was also an earlier fix rEULSb6fbb2cfc723: Fix undefined.toLowerCase is not a function in jquery.webfonts.js but it does not cover this case.