src/vs/base/browser/ui/tree/abstractTree.ts/AbstractTreeViewState/constructor
class AbstractTreeViewState:	protected constructor(state: IAbstractTreeViewState) {
		this.focus = new Set(state.focus);
		this.selection = new Set(state.selection);
		if (state.expanded instanceof Array) { // old format
			this.expanded = Object.create(null);
			for (const id of state.expanded as string[]) {
				this.expanded[id] = 1;
			}
		} else {
			this.expanded = state.expanded;
		}
		this.expanded = state.expanded;
		this.scrollTop = state.scrollTop;
	}