Fix bug where copying text inside a comment box wasn’t working

Change-Id: Ifc4866f82fab22db21c2494601ae4173e2c46af5
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
index 7e89f83..09b83b2 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js
@@ -355,6 +355,9 @@
     },
 
     _handleCopy: function(e) {
+      if (!e.target.classList.contains('content')) {
+        return;
+      }
       var text = this._getSelectedText(this._selectionSide);
       e.clipboardData.setData('Text', text);
       e.preventDefault();