src/vs/workbench/api/browser/mainThreadCustomEditors.ts/MainThreadCustomEditorModel/undo
class MainThreadCustomEditorModel:	private async undo(): Promise<void> {
		if (!this._editable) {
			return;
		}

		if (this._currentEditIndex < 0) {
			// nothing to undo
			return;
		}

		const undoneEdit = this._edits[this._currentEditIndex];
		this.change(() => {
			--this._currentEditIndex;
		});
		await this._proxy.$undo(this._editorResource, this.viewType, undoneEdit, this.isDirty());
	}