Question

I have a git repo on GitHub (demo), which has 3 branches:

• main : a default branch
• branch-modify : branched by main, contains a modifying commit to main
• branch-revert : branched by branch-modify, contains another commit reverting the modifying commit

On this repo, I made a PR with branch-modify against main and merged with a merged commit (#1).
Also I made another PR with branch-revert against main (#2).

I expected the second PR shows diff reverting the change by the modifying commit.
However, GitHub shows empty diff.

Why did the diff end up empty? If this is an intended behavior, what is the rational behind it?

Git graph of repo after the operations:

% git log --all --oneline --graph
*   97d2b9b (HEAD -> main, origin/main) Merge pull request #1 from nekomachi-touge/branch-modify
|
| | * 6d23f11 (origin/branch-revert, branch-revert) Revert "modify test.txt"
| |/
| * eaec7f6 (origin/branch-modify, branch-modify) modify test.txt
|/
* c1f094b add test.txt

Prior research

I found similar questions:

• git compare doesnt show any difference
• Github showing an incorrect diff between main branch and PR branch

Both suggest the difference between two-dot and three-dot diffs, but I don’t spot any difference in my case.

% git diff main..branch-revert
diff --git a/test.txt b/test.txt
index 55b5f1f..323fae0 100644
--- a/test.txt
+++ b/test.txt
@@ -1 +1 @@
-foofoo
+foobar
% git diff main...branch-revert
diff --git a/test.txt b/test.txt
index 55b5f1f..323fae0 100644
--- a/test.txt
+++ b/test.txt
@@ -1 +1 @@
-foofoo
+foobar

Also, I found the “Comparing changes” function produce diffs not converse if I compare main to branch-revert then swap the branches:
https://github.com/nekomachi-touge/github-revert-test-20240814/compare/main…branch-revert
https://github.com/nekomachi-touge/github-revert-test-20240814/compare/branch-revert…main

Background

In my work, I have an IaC repo controlling two clusters with active/standby-style redundancy.
To switch active cluster, I need to make a change in the repo.

I wanted to test the redundancy by making a PR then reverting it with another PR.
However, after merging the former PR, the diff of the latter PR became empty, which was totally unexpected.
Trying to reproduce and understand the issue led me into posting this question.