@Addshore No doubt about it. I did not want to create the impression that I consider it a valid approach to put that burden onto the end user (professional honor & UX-wise).
Happens too when executing only the tests inside /var/www/mediawiki/extensions/WikibaseLexeme/tests/phpunit/mediawiki
Locally reproduceable on WB+WBL masters
Will send the summary once the documentation contains the bits we added.
Mon, Aug 13
Fri, Aug 10
Now in wikibase master - let's keep an eye on this if it creates stable improvement. (not closing just yet)
@Addshore Did this ever happen after prod was re-deployed?
Thu, Aug 9
Patch now ensures that ResourceLoader provided the config value before using it. The waiting for the presence of ResourceLoader itself is a bit cumbersome - all suggestions welcome!
There appears to be an additional, unrelated flakyness in reading window.mw.config.get( 'LexemeLanguageCodePropertyId' ); possibly before resourceloader is fully finished.
With masters of core, wikibase, lexeme we seem to be able to reproduce it locally (sometimes; likely cache related).
Tried and traded the suggestion confirmation via enter for a click on the option. Seems to improve situation, but now other tests break (likely unrelated but red no less).
Wed, Aug 8
Unfortunately I could not come up with a completely different, more robust, approach of setting the value on the ComboBoxWidget.
I tried a couple of different ways of improving what was, making sure it is not the sheer speed at which the UI interactions are performed (the latest one going as far as not being actually fit for production). However waiting still does not seem to be enough which could point towards the mouse theory mentioned by Adam.