Core: Hide Scrollbar highlight

This patch hides the scrollbar overlay when the Scrollbar itself
gets hidden, for example on mac for documents that fit into the
viewport.

Fixes: QTCREATORBUG-28336
Change-Id: I2cb61affe38d0066a3d6f3ca012f97674b3d1539
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/coreplugin/find/highlightscrollbarcontroller.cpp

@@ -232,6 +232,12 @@ bool HighlightScrollBarOverlay::eventFilter(QObject *object, QEvent *event)
     case QEvent::ZOrderChange:
         raise();
         break;
+    case QEvent::Show:
+        show();
+        break;
+    case QEvent::Hide:
+        hide();
+        break;
     default:
         break;
     }