A combobox in a read only state makes it very difficult for the user to copy its value to the clipboard. It should be consistent and as easy as with the writeable combobox. You click inside, all the text gets selected and you can just Ctrl+C.
I would also get rid of the “stop” sign mouse cursor in this case. I think the fix does not solve that. Because it is then not easy to really select some text from the combo box and there is nothing to be “stopped” anyway.