Smooth scrolling

App/Resources/Tagger.js

@@ -150,8 +150,62 @@ class ElementTagger {
     }
 }
 
+class Scroller
+{
+    constructor() {
+        this.modelPositionY = 0;
+        this.modelPositionX = 0;
+    }
+
+    scrollBy(xOffset, yOffset) {
+        var initialPositionY = this.modelPositionY;
+        var initialPositionX = this.modelPositionX;
+        if (this.timerIdentifier == undefined) {
+            this.startTimer();
+
+            initialPositionY = window.scrollY;
+            initialPositionX = window.scrollX;
+        }
+
+        this.modelPositionY = initialPositionY + yOffset;
+        this.modelPositionY = Math.max(
+            0, Math.min(
+                this.modelPositionY, document.documentElement.scrollHeight - window.visualViewport.height
+            )
+        );
+
+        this.modelPositionX = initialPositionX + xOffset;
+        this.modelPositionX = Math.max(
+            0, Math.min(
+                this.modelPositionX, document.documentElement.scrollWidth - window.visualViewport.width
+            )
+        );
+    }
+
+    startTimer() {
+        this.timerIdentifier = setInterval(this.updateTimer, (30 / 1000), this);
+    }
+
+    stopTimer() {
+        clearInterval(this.timerIdentifier);
+        this.timerIdentifier = undefined;
+    }
+
+    updateTimer(self) {
+        const accelerationFactor = 0.05;
+        let positionOffsetY = (self.modelPositionY - window.scrollY) * accelerationFactor;
+        let positionOffsetX = (self.modelPositionX - window.scrollX) * accelerationFactor;
+        window.scrollBy(positionOffsetX, positionOffsetY);
+
+        if (Math.abs(positionOffsetX) <= 1.0 && Math.abs(positionOffsetY) <= 1.0) {
+            self.stopTimer();
+        }
+    }
+}
+
 (function() {
     let tagger = new ElementTagger();
+    let scroller = new Scroller();
 
     var tagAccum = [];
     document.addEventListener('keypress', (event) => {
@@ -185,18 +239,15 @@ class ElementTagger {
             let lowerKey = keyName.toLowerCase();
             let scrollAmount = event.shiftKey ? window.visualViewport.height : 30;
 
-            var scrollOptions = { behavior: 'smooth' }; // currently doesn't work with WebKit
             if (lowerKey == 'j') {
-                scrollOptions.top = scrollAmount;
+                scroller.scrollBy(0, scrollAmount);
             } else if (lowerKey == 'k') {
-                scrollOptions.top = -scrollAmount;
+                scroller.scrollBy(0, -scrollAmount);
             } else if (lowerKey == 'h') {
-                scrollOptions.left = -scrollAmount;
+                scroller.scrollBy(-scrollAmount, 0);
             } else if (lowerKey == 'l') {
-                scrollOptions.left = scrollAmount;
+                scroller.scrollBy(scrollAmount, 0);
             }
-            
-            window.scrollBy(scrollOptions);
         }
     });