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.