Set overflow-y: scroll for pdf.js viewer container (#5539)

sets overflow-y: scroll for the inner pdf.js viewer container, so that there's always space reserved for the vertical scrollbar, which means there's never a flicker at the point where the PDF height starts to overflow the container height.

GitOrigin-RevId: 6ec4621e723869810eff531c971ee26c4ed86638

services/web/frontend/stylesheets/app/editor/pdf.less

@@ -213,7 +213,7 @@
     }
     .pdfjs-viewer-inner {
       position: absolute;
-      overflow: auto;
+      overflow-y: scroll;
       width: 100%;
       height: 100%;
       -webkit-font-smoothing: initial;