Hi, @Tom !
Onsen UI automatically adds 20px gap on top of the screen if the app runs on Cordova.
But your app seems to already have a 20px white status bar for some reason, so you have to disable the feature (auto status bar fill) of Onsen UI. You just need to execute
ons.disableAutoStatusBarfill() in the beginning of your app.
Thank you for the information.
viewport-fit=cover takes effect only on iPhone X, I do not think it is related to this issue.
(Dec 22, 2017: Sorry, it seems that UIWebView automatically adds 20 px gap since iOS 11. You should add
viewport-fit=cover even if the app is running on iOS devices other than iPhone X)