Disable selection on comment when select across comment for Polymer 2

Change-Id: I2ea8e48586e7896a682a3f0e3c6f8152b9aaf478
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index bc8af9d..84a4d59 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -333,9 +333,9 @@
         user-select: text;
       }
 
-      /** Make comments selectable */
-      .selected-left ::slotted(gr-comment-thread[comment-side=left]),
-      .selected-right ::slotted(gr-comment-thread[comment-side=right]) {
+      /** Make comments selectable when selected */
+      .selected-left.selected-comment ::slotted(gr-comment-thread[comment-side=left]),
+      .selected-right.selected-comment ::slotted(gr-comment-thread[comment-side=right]) {
         -webkit-user-select: text;
         -moz-user-select: text;
         -ms-user-select: text;