Declining this task. Expansion of relative urls seems problematic. Even if we could provide the document context reliably, there'd be too many ways to trigger code without a proper context (API, Maintenance scripts etc.). Relative paths need to be given to the browser as-is and used there, if meant to be relative.

If not meant to be relative, they need to be adjusted in the calling code before reaching wfExpandUrl(). For expansion between two internal paths, we do have Wikimedia\RelPath which may proof useful.

@tstarling wrote at https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/87482/:

It's not technically correct to expand a path-relative URL as if it were relative to the server root -- it should be relative to the path of the current document. Presumably the difficulty of implementing path-relative URL expansion is the reason why it was originally omitted from the function.