Scenario:

The Azure App Service Extension uses ShowInputBox to prompt user for the name of the new web app to be created. Then it checks the name availability by trying to resolve the DNS name of the web app. This operation can take up to a few seconds depending on network speed.

Currently the ShowInputBox doesn't seem to support async validation function, which means we can only validate after ShowInputBox function is done and re-prompt if the DNS value turns out not available.

The optimum experience: ShowInputBox supports async validation, when the validation function is running, the input box shows a progress indicator and won't allow the value be accepted (pressing Enter won't close the box), ESC should still cancel the input regardless.