interfaces.php - Attempting to add a new gateway fails in several ways
When changing an interface to "Static IPv4", there is a button to "Add a New Gateway". On 2.2.x this was all dynamic/JS/AJAX style. Click the link, a form appeared, fill in the form, save and it added the entry to the gateways list/drop-down (and config.xml) without reloading the page.
On 2.3 clicking the "Add a New Gateway" button appears to submit the whole page which results in an input validation error.
The following input errors were detected:
A valid gateway must be specified.
Filling in the gateway form and then clicking "Add" also fails with the same error.