Let's say I run:
$ git clone https://gerrit.wikimedia.org/r/p/mediawiki/core.git w
And then I navigate to w/index.php in a Web browser. I get the following error message:
Error: LightnCandy class not defined
It's difficult for me to imagine a more unhelpful error message. The issue here (of course) is that I'm missing some Composer-related dependencies. Cloning the vendor repo is one "workaround."
However, we really should do better here. We should gracefully catch this error and output a sane, useful error message. Something like "hey, maybe you want to clone the vendor repo or read this wiki page about Composer" or something.