Option to position *cdkDragPreview under the cursor

[JohnKis]

Feature Description

When using custom preview templates with *cdkDragPreview, the preview is always positioned next to the cursor. This behaviour is different to using the default preview where the preview is positioned relative the pickup location.

I had a look in the code and removing the if shorthand at https://github.com/angular/components/blob/master/src/cdk/drag-drop/drag-ref.ts#L753 solves this. Checking the _getPointerPositionInElement method I couldn't find any reason why it wouldn't work with custom preview templates.

Use Case

This would be improve UX when using a custom preview template that has similar dimensions to the root element.

[crisbeto]

The reason why it was made this way is because we can't know ahead of time the size of the custom preview until it's actually in the DOM.

[JohnKis]

@crisbeto - Fair enough. I guess it makes more sense in cases where the preview size is known to be the same as the draggable element's. I opened a related feature request: #18177.

It could be an option that's disabled by default.

[crisbeto]

Closing since it should be handled by #18177.

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