[pdf file view] customized the position of the findbar (made it display above the highlighted search result, instead of covering it) (#6973)

frontend/src/css/pdf-file-view.css

@@ -34,6 +34,10 @@
   outline: none;
 }
 
+#findbar {
+  top: -3px;
+}
+
 .pdfViewer .page {
   position: relative;
   background: #fff;