previousIndex unexpectedly jumps with Angular Material drag/drop + cdk virtual scrolling

Question

Using Angular with Angular Material, I've created a sample app that features drag and drop lists, one of which which uses virtual scrolling. However, with certain items, the drop event's previousIndex value suddenly jumps to a different values. I'm baffled.

Steps to Reproduce

Drag the numbers 1 through 10 from the "To do" list to the "Done" list. When the card containing "10" is dropped, the wrong card is transferred, and previousIndex enabling that action jumps from 0 to 1 as seen in the console log.

Code

Stackblitz is here .

HTML

<div class="whole_thing" cdkDropListGroup>

  <div class="example-container">
    <h2>To do (CDK virtual scroll)</h2>

    <div
      cdkDropList
      [cdkDropListData]="todo"
      class="example-list"
      (cdkDropListDropped)="drop($event)">
      <cdk-virtual-scroll-viewport itemSize="62" class="example-viewport">
        <div class="example-box" *cdkVirtualFor="let item of todo" cdkDrag>{{item}}</div>
      </cdk-virtual-scroll-viewport>
    </div>
  </div>

  <div class="example-container">
    <h2>Done (regular list)</h2>

    <div
      cdkDropList
      [cdkDropListData]="done"
      class="example-list"
      (cdkDropListDropped)="drop($event)">
      <div class="example-box" *ngFor="let item of done" cdkDrag>{{item}}</div>
    </div>
  </div>

Typescript

import { Component, OnInit } from '@angular/core';
import {CdkDragDrop, moveItemInArray, transferArrayItem} from '@angular/cdk/drag-drop';

@Component({
  selector: 'app-thing',
  templateUrl: './thing.component.html',
  styleUrls: ['./thing.component.css']
})
export class ThingComponent implements OnInit {


  constructor() { }

  ngOnInit(): void {
  }

  todo = [
    '1',
    '2',
    '3',
    '4',
    '5',
    '6',
    '7',
    '8',
    '9',
    '10',
    '11',
    '12',
    '13',
    '14',
    '15',
    '16'
  ];

  done = [
    'Get up',
    'Brush teeth',
    'Take a shower',
    'Check e-mail',
    'Walk dog'
  ];

  drop(event: CdkDragDrop<string[]>) {
    if (event.previousContainer === event.container) {
      moveItemInArray(event.container.data, event.previousIndex, event.currentIndex);
    } else {
      transferArrayItem(event.previousContainer.data,
                        event.container.data,
                        event.previousIndex,
                        event.currentIndex);
    }
    //spread operator forces reload
    this.todo = [...this.todo]
    this.done = [...this.done]
    console.log(this.done)
    console.log(event)
  }

}

CSS

.example-container {
  width: 400px;
  max-width: 100%;
  margin: 0 25px 25px 0;
  display: inline-block;
  vertical-align: top;
}

.example-list {
  border: solid 1px #ccc;
  min-height: 60px;
  background: white;
  border-radius: 4px;
  overflow: hidden;
  display: block;
}

.example-box {
  height: 61px;
  /* padding: 20px 10px; */
  border-bottom: solid 1px rgb(192, 11, 11);
  color: rgba(0, 0, 0, 0.87);
  display: flex;
  flex-direction: row;
  align-items: center;
  justify-content: space-between;
  box-sizing: border-box;
  cursor: move;
  background: white;
  font-size: 14px;
}

.cdk-drag-preview {
  box-sizing: border-box;
  border-radius: 4px;
  box-shadow: 0 5px 5px -3px rgba(0, 0, 0, 0.2),
              0 8px 10px 1px rgba(0, 0, 0, 0.14),
              0 3px 14px 2px rgba(0, 0, 0, 0.12);
}

.cdk-drag-placeholder {
  opacity: 0;
}

.cdk-drag-animating {
  transition: transform 250ms cubic-bezier(0, 0, 0.2, 1);
}

.example-box:last-child {
  border: none;
}

.example-list.cdk-drop-list-dragging .example-box:not(.cdk-drag-placeholder) {
  transition: transform 250ms cubic-bezier(0, 0, 0.2, 1);
}

.example-viewport {
  height: 244px;
  width: 400px;
}

.example-item {
  height: 50px;
}

.whole_thing {
  width: 3000px;
}

.bigscroller {
  height: 600px;
  width: 100%;
}

Answer 1

The issue is that you are using cdk-virtual-scroll-viewport which is used to render huge lists. To do so the lists are sliced and only partial rendered and thats why your indeces are messed up.

A fix I thought of (not sure if its the best) is to pass the index given by the cdkVirtualFor as drag data and use that to handle the order.

<cdk-virtual-scroll-viewport itemSize="62" class="example-viewport">
  <div class="example-box" *cdkVirtualFor="let item of todo; let i = index;" cdkDrag [cdkDragData]="i">{{item}}</div>
</cdk-virtual-scroll-viewport>

and in the component

transferArrayItem(event.previousContainer.data,
                        event.container.data,
                        event.item.data,
                        event.currentIndex);

Answer 2

<cdk-virtual-scroll-viewport
formArrayName="items"
cdkDropList
[itemSize]="options.itemSize"
[perfectScrollbar]="config"
[cdkDropListDisabled]="!options.dragDrop"
(cdkDropListDropped)="onDragDrop($event)"
class="ps"
>
  <div
    *cdkVirtualFor="let item of itemsFiltered$ | async; let currentIndex = index"
    cdkDrag
    cdkDragLockAxis="y"
    [cdkDragData]="currentIndex"
    fxLayout="row"
    class="row"
    [class.drag]="options.dragDrop"
  >
...

onDragDrop(event: CdkDragDrop<AbstractControl[]>) {
    // Fix drag & drop indices inside virtual scroll
    const previousIndex = event.item.data;
    const adjustment = event.previousIndex - previousIndex;
    moveItemInArray(this.items.controls, previousIndex, event.currentIndex - adjustment);
    this.reindex();
    this.emitChanges();
  }