I'm not 100% sure if this is specifically a MediaWiki issue or an issue with a script somewhere, but:

• Go to https://test.wikipedia.org/w/index.php?title=Sandbox&action=edit
• Click on the "Show preview" button below the edit box

The page title then changes from Editing "Sandbox" to {{#ifeq:{{NAMESPACE}}|User | {{#switch:{{SUBPAGENAME}} |monobook.js|monobook.css|modern.js|modern.css|common.js|common.css=Editing {{BASEPAGENAME}}'s personal interface page |#default=Editing {{#titleparts: {{PAGENAME}} | 1 }}'s <!-- it's hard to identify the current page title if all we see is the user name and not the subpage name -->{{#ifeq: {{SUBPAGENAME}} | {{PAGENAME}} | user page | sub page "{{SUBPAGENAME}}" }} }} | Editing "$1" }}.

The interface message which contains the text is [[MediaWiki:Editing]], but my question is this: is whatever JavaScript code that generates the content when a preview is requested supposed to display the plain text from that page, or is it supposed to parse the result, then post it (then again, why does it even need to be changed from its initial value before submission in the first place)?

I get the feeling this might be an on-wiki script somewhere, but I'm putting this here just in case it isn't, or there's a MediaWiki core aspect to it (P.S. the regenerated title still has $1).