feat(drag-drop): Make it easier to know when a CdkDropList is [no longer] hovered

[SadiinsoSnowfall]

Feature Description

Currently, the CDK/Drag-Drop module is very opinionated about what it allows the developer to do: have multiple lists/containers that a dragged item will snap to. For example, the two events on CdkDropList that tell you the current container to which a dragged item will snap to are cdkDropListEntered and cdkDropListExited.

However, because of the snapping functionality built into the module from the start, the cdkDropListExited event only fires when the item changes its linked container, not when the container HTMLElement is no longer hovered over (what we would expect from dragEnter and dragLeave from the native browser API).

While this makes the implementation and management of snapping containers much easier, it only complicates the implementation of non-snapping logic. Mostly because there is no easy way to know when a cdkDropList is no longer hovered.

My proposal is to either (or both)

a) Introduce two new events in cdkDropList that would mirror the native dragEnter and dragLeave events and would be triggered when a dragged item enter/leave the area occupied by the container's HTML element (the event mimicking dragEnter would not constitute a duplicate of the already existing cdkDropListEntered one because it is possible to drag the mouse cursor in and out of the container area multiple times while keeping the dragged item bound to it if no other container was hovered over during the operation).

b) Introduce a new type named cdkHoverTarget for example, which would act as a non-snapping hover target (and possibly not a container). It would expose the two events reflecting dragEnter and dragLeave and would be used to know when the user has its mouse cursor over a specific element during a drag operation.

Use Case

One use of such a feature that I can think of would be in implementing a file manager: when dragging a file over a folder to move it into that folder, one would not expect the file preview to keep saying "move to " when the folder is no longer hovered over. This use case requires the developer to be able to know when a possible drop target is no longer hovered over, which is not trivial to achieve using the currently exposed events of the drag-drop module.

[angular-robot[bot]]

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[crisz]

Besides the feature request, is there any active workaround to detect when the container HTMLElement is no longer hovered over?