Fix thread-group styling with native shadow DOM

Change-Id: Iaa65ef7df689b41426cd6b426a4b1b948e8f7a7c
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 8f327aa..7f7ec72 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -35,7 +35,7 @@
       :host(.no-left) .sideBySide ::content .right:not([data-value]) + td {
         display: none;
       }
-      ::slotted(*) .thread-group {
+      .thread-group, ::slotted(*) .thread-group {
         display: block;
         max-width: var(--content-width, 80ch);
         white-space: normal;