Mecurial repos are only able to create a branch from default branch.

How to reproduce

• On a mercurial repo go to Branches tab.
• Click on "Create branch".
• The only "From branch" available is the branch set as default.

Workaround

1.

• Go to repo Settings.
• Set the default branch to the branch you intend to use.
• Create the branch from the correct one.
• Set the default branch back to the correct one.

2.

Create a new branch locally, add commits and push to remote:
hg push --new-branch