I’ve made a couple more cleanup fixes to the core search UI behavior, so namespace selections work more consistently, and have gone ahead and switched it in as the sole search interface on all Wikimedia wikis.
This means the LuceneSearch extension is officially obsolete. The MWSearch extension provides a back-end plugin for MediaWiki’s core search user interface, and all further front-end work should be done in core where it’ll benefit everybody.
I took the liberty of adapting the English Wikipedia’s JS to work with either case.
Please feel free to pass that fix on to other wikis.