@yildirimcagri-msft this looks like a bug?

This seems to be because the opened window is opened as a basic popup window and is not associated to a WebView2 and as a result the WebResourceRequest with the custom scheme cannot be handled by the app. For this to work, you'd need to associate a new WebView2 to the opened window by handling the NewWindowRequested event and setting NewWindow and then you can set a WebResourceRequested event handler on the new WebView2.

Thank you for the guidance. It sounds like the high level steps to get this to cooperate
in the NewWindowRequestedEventHandler,
HRESULT myNewWindowRequestedEventHandler(
ICoreWebView2* sender, ICoreWebView2NewWindowRequestedEventArgs* args)
are:

• get t(he deferal
  args->GetDeferral(&deferral);

• create a new web view controller
  HRESULT cwvRc = m_env->CreateCoreWebView2Controller(hWndParent,
  Callback(...)
  )
  What should the parent window be?
  I guess the originating web view's parent because the originating web view itself
  may close independently of the about to be created web view.

• use that controller to create a new web view

• setup the new web view's callbacks including the WebResourceRequested callback

• adjust the new web view's position and size as desired

• adjust the initiating NewWindowRequestedEventArgs argument via
  args->put_NewWindow(newWebView);
  args->put_Handled(TRUE);

• move the original NewWindowRequestedEventHandler call onward via
  deferral->Complete();

In CEF3 when we did a window.open() call a new Window was automatically created and the referenced URI content was shown within it. That new Window mapped logically to a new tab in a regular browser.
My approach above was close.
The parent window value above needs rethinking because I ended up with 2 web views in my main window. Oops.
Any thoughts on what the CEF3 world was doing for me automatically?

I ended up creating a new empty window as the parent for the new web view2 instance.