Snag

Press ~ on any project's page, pick the part that's wrong, and file a GitHub issue for that project, with a screenshot and where it was.

Try it here

Press ~ (outside a text field), then click a part of this page or drag over an area. ↑ and ↓ widen and narrow the pick, Esc cancels. Filing needs a super admin's sign-in.

Add it to a project

One line in the page's <head>, with the project's repo name:

<script src="https://snag.gowebsitedev.com/snag.js" data-project="repo-name" async></script>

Updating Snag updates every project that has it. The repo's README covers the options and Content-Security-Policy.