I fixed both the screen scroll issue and copy/look up button issue.
For some reason my config.xml file did not contain:
<preference name="DisallowOverscroll" value="true" />
Adding this back fixed the screen scrolling problem.
The copy/look up issue was resolved by adding this code to the css in index.html
I have no idea why this issue appeared after updating to iOS 11.3. But if it happens to you, start with these changes.
Remote previewing seems to have the exact same issue as what you have described on any of the mobile browser in iOS 11.3.1. Is there any lines I can edit with the caproj file? I'm on Steam C2 version though.