The "brief explanation" is what the commit message's subject line is for :) Just turn on "show `git blame` beside code" in your IDE, and you'll get all the brief explanations you could ever want!

...but no, to be serious for a moment: this isn't really a workable idea as-is, but it could be. It needs discoverability — mostly in not just throwing noise at you, so that you'll actually pay attention to the signal. If there was some way for your text editor or IDE to not show you all the `git blame` subject lines, but just the ones that were somehow "marked as non-obvious" at commit time, then we could really have something here.

Personally, I think commit messages aren't structured enough. Github had the great idea of enabling PR code-reviewers to select line ranges and annotate them to point out problems. But there's no equivalent mechanism (in Github, or in git, or in any IDE I know of) for annotating the code "at commit time" to explain what you did out-of-band of the code itself, in a way that ends up in the commit message.

In my imagination, text-editors and IDEs would work together with SCMs to establish a standard for "embeddable commit-message code annotations." Rather than the commit being one long block of text, `git commit -p` (or equivalent IDE porcelain) would take you through your staged hunks like `git reset -p` does; but for each hunk, it would ask you to populate a few form fields. You'd give the hunk a (log-scaled) rating of non-obviousness, an arbitrary set of /[A-Z]+/ tags (think "TODO" or "XXX"), an eye-catching one-liner start to the explanation, and then as much additional free-text explanation as you like. All the per-hunk annotations would then get baked into a markdown-like microformat that embeds into the commit message, that text-editors/IDEs could recognize and pull back out of the commit message for display.

And then, your text-editor or IDE would:

1. embed each hunk's annotation-block above the code it references (as long as the code still exists to be referenced — think of it as vertical, hunk-wise "show `git blame` beside code");

2. calculate a visibility score for each annotation-block based on a project-config-file-based, but user-overridable arbitrary formula involving the non-obviousness value, the tags, the file path, and the lexical identifier path from the syntax highlighter (the same lexical-identifier-path modern `git diff` gives as a context line for each diff-hunk);

3a. if the visibility score is > 1, then show the full annotation-block for the hunk by default;

3b. else, if the visibility score is > 0, then show the annotation-block folded to just its first line;

3c. else, hide the annotation-block (but you can still reveal the matching annotation with some hotkey when the annotated lines are focused.)

Of course, because this is just sourced from (let's-pretend-it's-immutable) git history, these annotation-block lines would be "virtual" — i.e. they'd be read-only, and wouldn't have line-numbers in the editor. If the text-editor wants to be fancy, they could even be rendered in a little sticky-note callout box, and could default to rendering in a proportional-width font with soft wrapping. Think of some hybrid of regular doc-comments, and the editing annotations in Office/Google Docs.

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...though, that's still not going as far as I'd really like. My real wish (that I don't expect to ever really happen) is for us to all be writing code as a series of literate codebase migrations — where your editor shows you the migration you're editing on the left, and the gestalt codebase that's generated as a result of running all migrations up to that one on the right, with the changes from the migration highlighted. You never directly edit the gestalt codebase; you only edit the migrations. And the migrations are what get committed to source-control — meaning that any code comments are there to be the literate documentation for the changes themselves; while the commit messages exist only to document the meta-level "editing process" that justifies the inclusion of the change.

Why? Because the goal is to structure the codebase for reading. Such a codebase would have one definitive way to learn it: just read the migrations like a book, front to back; watching the corresponding generated code evolve with each migration. If you're only interested in one component, then filter for just the migrations that make changes to it (`git log -S` style) and then read those. And if you, during development, realize you've just made a simple typo, or that you wrote some confusing code and later came up with a simpler way to express the same semantics — i.e. if you come up with code that "you wish you could have written from the start" — then you should go back and modify the earlier migration so that it's introduced there, so that new dev reading the code never has to see you introduce the bad version and then correct it, but instead just sees the good version from the start.

In other words: don't think of it as "meticulously grooming a commit history"; instead, think of it as your actual job not being "developer", but rather, as you (and all your coworkers) being the writers and editors of a programming textbook about the process of building program X... which happens to compile to program X.