Firstly apologies as I am a web developer, not really much of a Unity expert!
Unity Version: 5.5.3
Chrome: Version 62.0.3202.89 (Official Build) (64-bit)
Explanation:
I can’t select text once the Unity Viewer has loaded. It sounds like it might be this bug, but just checking if it is potentially something different as it’s more than a year old.
GIF:

This also seem relevant, and we are using the fix from here :
#if !UNITY_EDITOR && UNITY_WEBGL
WebGLInput.captureAllKeyboardInput = true;
#endif
but that doesn’t seem to fix the problem. Do you think upgrading Unity might help?