fix(modal): overlay-zindex is not changed in back direction

fixes #9409

src/components/modal/test/basic/app.module.ts

@@ -441,6 +441,9 @@ export class ContactUs {
       <p>
         <button ion-button (click)="openActionSheet()">Open Action Sheet</button>
       </p>
+      <p>
+        <button ion-button (click)="openModal()">Open same modal</button>
+      </p>
       <div f></div><div f></div><div f></div><div f></div><div f></div><div f></div><div f></div><div f></div><div f></div><div f></div>
       <div f></div><div f></div><div f></div><div f></div><div f></div><div f></div><div f></div><div f></div><div f></div><div f></div>
       <ion-list>
@@ -460,8 +463,9 @@ export class ModalFirstPage {
     public app: App,
     public actionSheetCtrl: ActionSheetController,
     public toastCtrl: ToastController,
-    public alertCtrl: AlertController) {
+    public alertCtrl: AlertController,
-
+    public modalCtrl: ModalController,
+  ) {
     for (let i = 0; i < 50; i++) {
       this.items.push({
         value: (i + 1)
@@ -550,6 +554,10 @@ export class ModalFirstPage {
     this.called.ionViewDidLeave++;
   }
 
+  openModal() {
+    this.modalCtrl.create(ContactUs).present();
+  }
+
   openActionSheet() {
     let actionSheet = this.actionSheetCtrl.create({
       buttons: [

src/navigation/nav-util.ts

@@ -58,7 +58,9 @@ let portalZindex = 9999;
 export function setZIndex(nav: NavControllerBase, enteringView: ViewController, leavingView: ViewController, direction: string, renderer: Renderer) {
   if (enteringView) {
     if (nav._isPortal) {
+      if (direction === DIRECTION_FORWARD) {
         enteringView._setZIndex(nav._zIndexOffset + portalZindex, renderer);
+      }
       portalZindex++;
       return;
     }