Merge pull request #5208 from CodingFabian/debounce-scroll

Debounces scroll events in web viewer.

web/viewer.js

@@ -274,7 +274,14 @@ var PDFView = {
   watchScroll: function pdfViewWatchScroll(viewAreaElement, state, callback) {
     state.down = true;
     state.lastY = viewAreaElement.scrollTop;
-    viewAreaElement.addEventListener('scroll', function webViewerScroll(evt) {
+    state.rAF = null;
+    viewAreaElement.addEventListener('scroll', function debounceScroll(evt) {
+      if (state.rAF) {
+        return;
+      }
+      // schedule an invocation of webViewerScrolled for next animation frame.
+      state.rAF = window.requestAnimationFrame(function webViewerScrolled() {
+        state.rAF = null;
         if (!PDFView.pdfDocument) {
           return;
         }
@@ -288,6 +295,7 @@ var PDFView = {
         // else do nothing and use previous value
         state.lastY = currentY;
         callback();
+      });
     }, true);
   },