It actually is simple to know who contributed every character in a line of code and when. That’s one of the biggest advantages of git … See git … the underlying version control system used by GitHub.
See: https://github.com/Jermolene/TiddlyWiki5/blame/master/licenses/cla-individual.md#L1
Most editors that are aware of git have a possibility to show you the “commit history” of every single line in the code.
This mechanism makes it possible to tell what exactly changed between TW versions and which change caused an issue. …