There's no particular reason to allow the user to edit the clone URI field in Diffusion; editing it has no meaning and if you fat finger the keyboard, it's quite possible that the user will either accidentally clear and/or modify the URI before copying (bit me this morning).

Adding a readonly attribute to the input field allows the same benefit (URI is easily selectable) while preventing such accidental input. Fixes T4246.

Verified that the desired behavior is present in both Chrome, Safari, and Firefox. Field remains selectable with one click, but field is not editable.