use requestAnimationFrame for cleaner progress bar updates

static/js/upload.js

@@ -31,6 +31,7 @@ let progressSize;
 let progressRate;
 let progressEta;
 let progressBar;
+let animationID = null;
 
 window.addEventListener('beforeunload', (event) => {
     if (socket && socket.readyState !== 3) {
@@ -291,7 +292,9 @@ function sendData() {
                 if (timestamps.length > SAMPLE_WINDOW) { timestamps.shift(); }
             }
 
-            updateProgress();
+            if (animationID === null) {
+                animationID = requestAnimationFrame(updateProgress);
+            }
         } else {
             fileIndex += 1;
             byteIndex = 0;
@@ -300,6 +303,8 @@ function sendData() {
 }
 
 function updateProgress() {
+    animationID = null;
+
     let percentage;
     if (totalBytes === 0) {
         percentage = "0%";