Edit the commit title and message when merging a pull request

[ilyaliao]

Problem

When you merge from T3 Code you can't see or change the commit the merge will create. The pull request panel asks for a second click on Confirm merge?, and the pull request row in thread details asks Merge pull request? in a dialog. Neither shows the message. GitHub's own merge box shows a commit title and a description, and you can edit both before you confirm.

This hurts most on squash merges. GitHub builds the default message from the pull request's commits or its description, and when an agent wrote those the message is often long. To land a clean commit I have to leave T3 Code and merge on GitHub.

T3 Code already changes this message once. Remove agent credits when merging (#16192) rewrites it on the way out, but you never see the result before it lands.

I searched issues, pull requests, and Ideas for an existing request and found none.

Proposal

Let you edit the commit title and message before a merge commit or a squash merge on GitHub, in a dialog with Commit title and Commit message fields. They start with the title and message GitHub would use, already without agent credits when that setting is on. If you don't touch them, the merge behaves exactly as it does today.

The steps before it stay as they are on main. In the pull request panel you still click the merge button and then Confirm merge?. That second click opens the dialog instead of merging, and the button in the dialog merges. In thread details the existing Merge pull request? dialog shows the fields.

Text you change is used as written. The merge is tied to the commit the message was shown for, so it fails if the pull request gets new commits in between. Your text is kept, so you can check it and confirm again.

There is no dialog where GitHub ignores a custom message: rebase merges and repositories with a merge queue. There the second click merges, as it does today. Native stack merges keep GitHub's message. Other hosts stay as they are. Support is a capability flag on the host, so another host can opt in later. Mobile keeps its own confirmation and GitHub's default message.

What I have so far

I already have a working implementation, and it works in my local testing. This is the dialog for a squash merge, filled with GitHub's default message:

I'm happy to finish this and open a PR if you want it.