feat(cdk/drag-drop): allow drag to be canceled after dragged over list

[andrewseguin]

Feature Description

Items actively dragged from one list to another, but without releasing the drag, cannot be canceled. The list changes immediately and the user cannot undo the action. This issue tracks the request to enable an API that lets dragging be canceled, e.g. by undo'ing the list changes (or not change the lists at all) if the item is not dropped onto a drop container

Use Case

No response

[pete-mcwilliams]

In our use case we have an enter predicate stoping the user from dragging to a list. If the user drops when mouseover that list then the dragged item will be inserted into wherever the last valid position in any list was, it just does not feel correct.

Drag 'brush teeth' over 'to do' and drop on the 'Can not be inserted' listed. brush teeth will be added to the todo list.
https://stackblitz.com/edit/xssa9h25?file=src%2Fexample%2Fcdk-drag-drop-connected-sorting-example.html,src%2Fexample%2Fcdk-drag-drop-connected-sorting-example.ts

[pete-mcwilliams]

From the other thread adding the following to the top of the drop function works well enough for my needs.

    if (!event.isPointerOverContainer) {
      return;
    }