Not that Parsoid will also append querystrings with delimiter to the end, like ?lang=de

the legacy parser will generate full absolute URLs in its output

See core's Title::getLocalURL()

I vaguely recall this issue being well(?) known, but the Parsoid output requires a particular URL setup.

There are some notes about this in T270219 and T261711.

It looks like this is more-or-less fixed in Parsoid (but see T350952) in so far as SiteConfig::relativeLinkPrefix() in core jumps through extreme hoops in order to generate a base href and a relativeUrlPrefix() which work even for wikis with ?title= URL setups. But this probably breaks VE (T270219) which still only looks for & strips the basic ./ prefix and gets confused when Parsoid generates hrefs with (for example) ./index.php?title= as the "relative link prefix" instead of ./.

Change 975081 had a related patch set uploaded (by C. Scott Ananian; author: C. Scott Ananian):

[mediawiki/core@master] [parsoid] Fix Parsoid relative links, take 2

https://gerrit.wikimedia.org/r/975081

Change 975081 abandoned by C. Scott Ananian:

[mediawiki/core@master] [parsoid] Fix Parsoid relative links, take 2

Reason:

Abandoned in favor of I60165a9946a35cfb42a78ed2f833c34570fefffc

https://gerrit.wikimedia.org/r/975081