Cannot search for merge requests by target branch when using a relative URL

Summary

Reproduced in GitLab 18.2.1-ee

If GitLab is using a relative URL, it is not possible to filter the list of merge requests by target branch.

Steps to reproduce

1. Setup GitLab with a relative URL.
2. Navigate to any project with merge requests and attempt to filter by Target Branch. You will see a There was a problem fetching branches error

Example Project

N/A

What is the current bug behavior?

When attempting to filter merge requests by Target Branch, you will see a There was a problem fetching branches error.

What is the expected correct behavior?

When attempting to filter merge requests by Target Branch, you should see a list of branches.

Relevant logs and/or screenshots

If we take a look at the Network tab, we can see a request is made to https://gitlab.example.com/-/autocomplete/merge_request_target_branches.json?project_id=xxxx which returns a 404. The URL is missing the relative path eg. https://gitlab.example.com/relative-path/-/autocomplete.....

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

Patch release information for backports

If the bug fix needs to be backported in a patch release to a version under the maintenance policy, please follow the steps on the patch release runbook for GitLab engineers.

Refer to the internal "Release Information" dashboard for information about the next patch release, including the targeted versions, expected release date, and current status.

High-severity bug remediation

To remediate high-severity issues requiring an internal release for single-tenant SaaS instances, refer to the internal release process for engineers.