Would you like to have your pages hosted on http://localhost:63342 by default despite having http://test.mydomain.com set up as a deployment server?
When you are using Open in browser action, or preview a file in browser using browsers bar in editor, or create a new Run/Debug configuration from .html file right-click menu, WebStorm uses URL from current Deployment entry (which is marked as Default) to build full URL. If you have no deployment entries defined, then built-in web server (http://localhost:63342) is used.
There is a feature request to optionally use built-in webserver for previewing files in browser regardless of deployment settings, WEB-18958.
For now, I can suggest the following workarounds:
- Make your deployment entry non-default (right-click,
Stop using as default)
Note that it will break auto-deployment (if you have it set up)

- or, use Run configuration with the built-in server URL for running your application. But, as run configurations created from right-click menu always have the default server URL pre-configured, you need to edit created configurations in
Run | Edit configurations... dialog, making sure that the built-in sderver URL is specified there instead of your deployment server URL.