Steps to reproduce:
1. Go to a page in a namespace where VE is not enabled.
2. Append `?veaction=edit` to the URL.
Expected:
Either nothing should happen or the URL should be treated as explicit opt-in to VE and allow editing in VE without problems.
Actual:
The actual result depends on your settings:
1. If you have NWE enabled: You are editing the HTML (not the wikitext) in source mode.
2. If you never edited before: You are shown a welcoming dialog without any button to close.
3. If neither of the above: You edit in VE, but with a few bugs. E.g. when reviewing your changes there will be an additional loading bar that doesn't go away.
Neither of the above is really surprising, the URL tries to set `visual` as `defaultMode`, but that mode isn't available, so you'll either end up with `source` as mode (case 1), or `undefined` (which behaves like `visual` but with the issues 2 or 3).