Hi @seav, thanks for taking the time to report this!

Can you please use Special:MovePage with debug=true (see https://www.mediawiki.org/wiki/Help:Locating_broken_scripts for some info) and post the complete JavaScript console error and also the exact location where it is triggered?

Hmmm. When I add the debug=true parameter to the URL, the problem disappears and there are no errors in the JS console.

Scratch that. The problem still exists but the form fields show for a much longer time before disappearing.

Here is the console log:

[jquery.js:3819:18]
jQuery.Deferred exception: invalid language tag: cbk-zam MwWidgetsTitleWidget@https://cbk-zam.wikipedia.org/w/resources/src/mediawiki.widgets/mw.widgets.TitleWidget.js?7d5dd:64:4
MwWidgetsTitleInputWidget@https://cbk-zam.wikipedia.org/w/resources/src/mediawiki.widgets/mw.widgets.TitleInputWidget.js?6bb5d:34:26
MwWidgetsComplexTitleInputWidget@https://cbk-zam.wikipedia.org/w/resources/src/mediawiki.widgets/mw.widgets.ComplexTitleInputWidget.js?685ff:27:16
OO.ui.Element.static.unsafeInfuse@https://cbk-zam.wikipedia.org/w/resources/lib/ooui/oojs-ui-core.js?3ed96:819:8
OO.ui.Element.static.infuse@https://cbk-zam.wikipedia.org/w/resources/lib/ooui/oojs-ui-core.js?3ed96:674:33
OO.ui.infuse@https://cbk-zam.wikipedia.org/w/resources/lib/ooui/oojs-ui-core.js?3ed96:331:30
@https://cbk-zam.wikipedia.org/w/resources/src/mediawiki.misc-authed-ooui/special.movePage.js?a472a:11:9
mightThrow@https://cbk-zam.wikipedia.org/w/resources/lib/jquery/jquery.js?6fb03:3535:29
resolve/</process<@https://cbk-zam.wikipedia.org/w/resources/lib/jquery/jquery.js?6fb03:3603:12
 undefined

[mw.widgets.TitleWidget.js:64:4]
RangeError: invalid language tag: cbk-zam

I tried the action here:
https://cbk-zam.wikipedia.org/wiki/Especial:MoverP%C3%A1gina/Dimasalang,_Masbate?debug=true

With the debug=true, I can move pages as long as I am able to submit the form before the namespace and title form fields disappear.

Thanks! That line is https://phabricator.wikimedia.org/source/mediawiki/browse/master/resources/src/mediawiki.widgets/mw.widgets.TitleWidget.js$63

I assume that's an OOUI widget but I'm not sure.

Please don't tag the library about problems with it use.

In T228510#5350178, @Jdforrester-WMF wrote:

Please don't tag the library about problems with it use.

Ah, sorry then. I thought the file might be part of the actual library codebase, but looks like it is not... :)

Change 587688 had a related patch set uploaded (by Nikerabbit; owner: Nikerabbit):
[mediawiki/core@master] Fix Special:MovePage in cbk-zam.wikipedia.org

https://gerrit.wikimedia.org/r/587688

Change 587688 merged by jenkins-bot:
[mediawiki/core@master] TitleWidget: Account for non-BCP47 language codes in Intl.Collator

https://gerrit.wikimedia.org/r/587688

Tested in production.