Add a threshold for viewport dragging (#600)

include/rfb.js

@@ -614,14 +614,18 @@ var RFB;
             if (this._viewportDragging) {
                 var deltaX = this._viewportDragPos.x - x;
                 var deltaY = this._viewportDragPos.y - y;
-                this._viewportDragPos = {'x': x, 'y': y};
 
-                // if there is actually viewport move, set the HasMoved flag to true
-                 if (deltaX != 0 || deltaY != 0) {
-                     this._viewportHasMoved = true;
-                 }
+                // The goal is to trigger on a certain physical width, the
+                // devicePixelRatio brings us a bit closer but is not optimal.
+                var dragThreshold = 10 * window.devicePixelRatio;
 
-                this._display.viewportChangePos(deltaX, deltaY);
+                if (this._viewportHasMoved || (Math.abs(deltaX) > dragThreshold ||
+                                               Math.abs(deltaY) > dragThreshold)) {
+                    this._viewportHasMoved = true;
+
+                    this._viewportDragPos = {'x': x, 'y': y};
+                    this._display.viewportChangePos(deltaX, deltaY);
+                }
 
                 // Skip sending mouse events
                 return;