Bug: Suggestion block button missing from toolbar in PR line comments

[alexwforsythe]

Expected behavior

When adding a line comment to a PR from the Files changed tab, there should be a ± button in the comment form Markdown toolbar that inserts a "suggestion" block containing the contents of the line(s) I'm commenting on.

Source: Review changes / Comment on a PR / Adding line comments to a pull request, step 6

Actual behavior

There is no ± button in the Markdown toolbar when adding line comments.

Single line:

Multiple lines:

The button only appears when replying to existing comments:

Extra info

• I'm not using any custom stylesheets or extensions that alter the behavior of GitHub
• The screenshots were taken from interactions on Added graphql language alexwforsythe/code-blocks#185
• I'd also like to request that the button appear when adding line comments from the Commits tab, which apparently isn't supported according to No suggestion button while clicking on a commit of the pull request #7128

[svenja-haas]

This button is also nonexistent for me in the "files changed" tab. No idea why.

Edit: Sooo it was because the PR was merged while I was reviewing, so I didn't notice. Would prefer it to be greyed out with a tooltip for more clarity in this case.