"Delete source branch after merging" is not refreshed after merging a pull request with the same source branch in a different tab

Summary

After upgrading to 5.8, when attempting to merge a pull request the "Delete source branch after merging" checkbox is disabled in the following conditions:

• a pull request with the same source branch already existed
• the user opened both pull requests at the same time

Steps to Reproduce

1. Create a branch (e.g. feature/test_branch2)
2. Create a pull request from feature/test_branch2 to develop
3. Create a pull request from feature/test_branch2 to master
4. Open both pull requests in the browser
5. Merge the pull request from feature/test_branch2 to develop. The "Delete source branch after merging" is correctly disabled in the merge dialog.
6. Without refreshing the previously opened page, attempt to merge the pull request from feature/test_branch2 to master

Expected Results

The "Delete source branch after merging" is enabled.

This is a screenshot of the behaviour in 5.7.1:

Actual Results

The "Delete source branch after merging" is disabled.

When hovering over it the "You can not delete the branch because there are open pull requests with the branch as the source" is displayed.

Workaround

Refresh / reopen the page before hitting the Merge button.