feat(module:modal): smart to determine whether to add padding-right

close NG-ZORRO#1422

components/modal/nz-modal.component.ts

@@ -414,13 +414,23 @@ export class NzModalComponent<T = any, R = any> extends NzModalRef<T, R> impleme
     const openModals = this.modalControl.openModals;
 
     if (openModals.length + plusNum > 0) {
-      this.renderer.setStyle(this.document.body, 'padding-right', `${this.nzMeasureScrollbarService.scrollBarWidth}px`);
-      this.renderer.setStyle(this.document.body, 'overflow', 'hidden');
-    } else {
+      if (this.hasScrollBar(this.document.body)) { // Adding padding-right only when body's scrollbar is shown up
+        this.renderer.setStyle(this.document.body, 'padding-right', `${this.nzMeasureScrollbarService.scrollBarWidth}px`);
+        this.renderer.setStyle(this.document.body, 'overflow', 'hidden');
+      }
+    } else { // NOTE: we need to always remove the padding due to the scroll bar may be disappear by window resizing before modal closed
       this.renderer.removeStyle(this.document.body, 'padding-right');
       this.renderer.removeStyle(this.document.body, 'overflow');
     }
   }
+
+  /**
+   * Check whether one element is able to has the scroll bar
+   * Exceptional Cases: users can show the scroll bar by their own permanently (eg. overflow: scroll)
+   */
+  private hasScrollBar(element: HTMLElement): boolean {
+    return element.scrollHeight > element.clientHeight;
+  }
 }
 
 ////////////