--- a/dom/browser-element/BrowserElementPanning.js
+++ b/dom/browser-element/BrowserElementPanning.js
@@ -236,19 +236,20 @@ const ContentPanning = {
     KineticPanning.record(delta, evt.timeStamp);
 
     // There's no possibility of us panning anything.
     if (!this.scrollCallback) {
       return;
     }
 
     let isPan = KineticPanning.isPan();
-    if (!isPan) {
-      // If panning distance is not large enough, both BES and APZC
-      // should not perform scrolling
+    if (!isPan && this.detectingScrolling) {
+      // If panning distance is not large enough and we're waiting to
+      // see whether we should use the sync scroll fallback or not,
+      // don't attempt scrolling.
       return;
     }
 
     let isScroll = this.scrollCallback(delta.scale(-1));
 
     if (this.detectingScrolling) {
       this.detectingScrolling = false;
       // Stop async-pan-zooming if the user is panning the subframe.
@@ -257,25 +258,27 @@ const ContentPanning = {
         Services.obs.notifyObservers(docShell, 'cancel-default-pan-zoom', null);
       } else {
         // Let AsyncPanZoomController handle the scrolling gesture.
         this.scrollCallback = null;
         return;
       }
     }
 
-    // If a pan action happens, cancel the active state of the
+    // If we've detected a pan gesture, cancel the active state of the
     // current target.
     if (!this.panning && isPan) {
       this.panning = true;
       this._resetActive();
       this._activationTimer.cancel();
     }
 
     if (this.panning) {
+      // Only do this when we're actually executing a pan gesture.
+      // Otherwise synthetic mouse events will be canceled.
       evt.stopPropagation();
       evt.preventDefault();
     }
   },
 
   // nsITimerCallback
   notify: function cp_notify(timer) {
     this._setActive(this.pointerDownTarget);