src/vs/workbench/browser/parts/editor/editorDropTarget.ts/DropOverlay/findSourceGroupView
class DropOverlay:	private findSourceGroupView(): IEditorGroupView | undefined {

		// Check for group transfer
		if (this.groupTransfer.hasData(DraggedEditorGroupIdentifier.prototype)) {
			const data = this.groupTransfer.getData(DraggedEditorGroupIdentifier.prototype);
			if (Array.isArray(data)) {
				return this.accessor.getGroup(data[0].identifier);
			}
		}

		// Check for editor transfer
		else if (this.editorTransfer.hasData(DraggedEditorIdentifier.prototype)) {
			const data = this.editorTransfer.getData(DraggedEditorIdentifier.prototype);
			if (Array.isArray(data)) {
				return this.accessor.getGroup(data[0].identifier.groupId);
			}
		}

		return undefined;
	}