The logic in webroot/rsrc/js/application/differential/behavior-user-select.js to make text unselectable makes inline comments unselectable in Unified diff view. In side-by-side view, both inline comments and code are selectable. It is often useful to be able to copy/paste text from other's inline comments.
Phabricator version 067d12d7168a580bce695cb26331df785f86c2e9, in Chrome 53.0.2785.116.