src/vs/workbench/browser/parts/views/treeView.ts/CustomTreeViewDragAndDrop/onDragOver
class CustomTreeViewDragAndDrop:	onDragOver(data: IDragAndDropData, targetElement: ITreeItem, targetIndex: number, originalEvent: DragEvent): boolean | ITreeDragOverReaction {
		return { accept: true, bubble: TreeDragOverBubble.Down, autoExpand: true };
	}