Ensure side-by-side diff lines don't collapse to 0px height

Bug: Issue 5091
Change-Id: I647205b2d1c05ddc0c871647331fbe3f773c04b5

polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html

@@ -138,6 +138,10 @@ limitations under the License.
       .content.remove.lightHighlight {
         background-color: var(--light-remove-highlight-color);
       }
+      .content .contentText:after {
+        /* Newline, to ensure all lines are one line-height tall. */
+        content: '\A';
+      }
       .contextControl {
         background-color: #fef;
         color: #849;