Windows source code now lives in git, and it has some incredible scaling challenges. 300GiB of files in HEAD! https://blogs.msdn.microsoft.com/bharry/2017/05/24/the-largest-git-repo-on-the-planet/
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On the challenge of writing accurate source spans on Unicode source code: https://reedmullanix.com/posts/unicode-source-spans.html
Also (see footnotes) a fair number of LSP clients assume UTF-8 despite early versions of LSP mandating UTF-16!
Over a sufficiently long time horizon, all code you write is legacy code.
Ooh, difftastic is now listed on the official git website! https://git-scm.com/tools/command-line