fix(nav): improve reliability of swipe back gesture when quickly swip…

…ing back

core/src/components/nav/nav.tsx

@@ -31,7 +31,7 @@ import { VIEW_STATE_ATTACHED, VIEW_STATE_DESTROYED, VIEW_STATE_NEW, convertToVie
 export class Nav implements NavOutlet {
   private transInstr: TransitionInstruction[] = [];
   private sbAni?: Animation;
-  private animationEnabled = true;
+  private gestureOrAnimationInProgress = false;
   private useRouter = false;
   private isTransitioning = false;
   private destroyed = false;
@@ -869,7 +869,36 @@ export class Nav implements NavOutlet {
     // or if it is a portal (modal, actionsheet, etc.)
     const opts = ti.opts!;
 
-    const progressCallback = opts.progressAnimation ? (ani: Animation | undefined) => (this.sbAni = ani) : undefined;
+    const progressCallback = opts.progressAnimation
+      ? (ani: Animation | undefined) => {
+          /**
+           * Because this progress callback is called asynchronously
+           * it is possible for the gesture to start and end before
+           * the animation is ever set. In that scenario, we should
+           * immediately call progressEnd so that the transition promise
+           * resolves and the gesture does not get locked up.
+           */
+          if (ani !== undefined && !this.gestureOrAnimationInProgress) {
+            this.gestureOrAnimationInProgress = true;
+            ani.onFinish(
+              () => {
+                this.gestureOrAnimationInProgress = false;
+              },
+              { oneTimeCallback: true }
+            );
+
+            /**
+             * Playing animation to beginning
+             * with a duration of 0 prevents
+             * any flickering when the animation
+             * is later cleaned up.
+             */
+            ani.progressEnd(0, 0, 0);
+          } else {
+            this.sbAni = ani;
+          }
+        }
+      : undefined;
     const mode = getIonMode(this);
     const enteringEl = enteringView.element!;
     const leavingEl = leavingView && leavingView.element!;
@@ -1008,15 +1037,16 @@ export class Nav implements NavOutlet {
 
   private canStart(): boolean {
     return (
+      !this.gestureOrAnimationInProgress &&
       !!this.swipeGesture &&
       !this.isTransitioning &&
       this.transInstr.length === 0 &&
-      this.animationEnabled &&
       this.canGoBackSync()
     );
   }
 
   private onStart() {
+    this.gestureOrAnimationInProgress = true;
     this.pop({ direction: 'back', progressAnimation: true });
   }
 
@@ -1028,10 +1058,9 @@ export class Nav implements NavOutlet {
 
   private onEnd(shouldComplete: boolean, stepValue: number, dur: number) {
     if (this.sbAni) {
-      this.animationEnabled = false;
       this.sbAni.onFinish(
         () => {
-          this.animationEnabled = true;
+          this.gestureOrAnimationInProgress = false;
         },
         { oneTimeCallback: true }
       );
@@ -1055,6 +1084,8 @@ export class Nav implements NavOutlet {
       }
 
       this.sbAni.progressEnd(shouldComplete ? 1 : 0, newStepValue, dur);
+    } else {
+      this.gestureOrAnimationInProgress = false;
     }
   }