Thank you for your post.
I am unable to replicate the issue using Chrome 54 under macOS Sierra 10.12.1. This is what I have done:
1. Using Chrome, I accessed the Web Direct home page for a FileMaker Server 15 machine.
2. I opened the FMServer_Sample.fmp12.
3. In the first record, I highlighted the entire contents of the Task field ("Site Map Sketch"), and copied it to the clipboard.
4. I put my cursor at the end of the Description field and successfully pasted the text.
Let me know what I am doing differently than you so I can replicate the issue.
Did you try with the contextual menu? The command/ctrl keys work for copy/paste, but the contextual menu is disabled. It works in other web forms, and even on the web direct login. It doesn't work in web direct text fields. I tried in Chrome on Mac/Win, IE Win, Safari Mac.
My apologies. I misread your initial post.
In Web Direct, the contextual menus only work for Container fields and Web Viewers. I am also unable to locate this information in the documentation, so I have asked this information be added to the documentation, and I have also put in a request to implement this in a future release.