In the starter template for publishing a jekyll-based site to Github pages using the new Actions infrastructure, it shows how you can autodetect the correct publishing URL:

As the comment suggests, this results in just "/" for the main site published to a domain (e.g. example.com) and to something like /subfolder/ for subprojects.

This is not what Jekyll expects; in fact the default configuration has baseurl be the empty string: https://jekyllrb.com/docs/configuration/default/

If you have a site that uses baseurl as a prefix to support separate deployments, the URLs generated will now start with two leading slashes rather than function as expected.

I see two ways to resolve this:

1. update the "Setup pages" action to reflect the correct baseurl which would be a breaking change, or
2. change the starter workflow, doing one or both of the following
   • Add a note that --baseurl … needs to be left out for a main site, or
   • Remove the --baseurl from the example altogether, make another workflow for subprojects specifically

I probably missed a cleaner solution, but this would at least work.