src/vs/workbench/browser/parts/compositeBar.ts/CompositeDragAndDrop/getTargetIndex
class CompositeDragAndDrop:	private getTargetIndex(targetId: string | undefined, before2d: Before2D | undefined): number | undefined {
		if (!targetId) {
			return undefined;
		}

		const items = this.getItems();
		const before = this.targetContainerLocation === ViewContainerLocation.Panel ? before2d?.horizontallyBefore : before2d?.verticallyBefore;
		return items.filter(o => o.visible).findIndex(o => o.id === targetId) + (before ? 0 : 1);
	}