Steps to reproduce:
- Create a userpage ([[User:Example]]). Create a /Sandbox in your userspace ([[User:Example/Sandbox]]).
- Open the userpage in VisualEditor.
- Add a relative link to the subpage: /Sandbox. This is easily done by opening the link tool and typing "/Sandbox" into the box.
- Note that the resulting link is red, even though you have already created the subpage.
- Click the link in the context menu to open it in a new tab. The link opened is to a page in the mainspace: /Sandbox.
- Save the page. Notice that the link is now blue, and leads to the subpage.
Expectation: The link in VisualEditor and the link when the page has been saved will lead to the same page.
(Search has the same "problem": if you search for /Sandbox, it will offer to let you create a page with that title, including the initial slash.)