Hi,
I moved your post into GAS as it's more a GAS question I guess.
When you pass from GAS 2.4x to GAS 2.5x version, the default mode changes: in 2.4x, application is by default displayed in AJAX mode while it is HTML5 mode since 2.5x.
So in your case, I think that issue occurs in HTML5 mode but not in AJAX mode. Can you confirm this by modifying the display mode in 2.4x and 2.5x as a test ?
To force a mode, just add the following "OutputMap" at the end of your application url:
http://xxxxxxx?OutputMap=DUA_AJAX
http://xxxxxxx?OutputMap=DUA_HTML5
Do you still get the issue in 2.50 + AJAX ?
Also, please confirm your expected behavior: you do a right-click in a textedit in display mode and a menu option allows you to select all the field content, then you can do a copy/paste of all the content ?
Best regards,
Guney