The first time you tap a text field in an in-app browser, the background of the app (behind the browser) briefly appears.
Create an HTML page to load in the in-app browser, with a text field, and store it as www/sample.html.
Now launch this on an iPhone simulator (I tried iPhone 6 and X) running iOS 11.2, attach a Safari debugger, and open the page in an in-app browser, e.g. window.open("sample.html", "_blank");
(In the attached app, you can just tap the Cordova icon to open it.)
Then tap the text field. You'll see an error in the Xcode logs (attachment).
In addition, the background of the app (including the Cordova icon) will briefly flash.