src/vs/platform/windows/electron-main/windowsMainService.ts/WindowsMainService/getRestoreWindowsSetting
class WindowsMainService:	private getRestoreWindowsSetting(): RestoreWindowsSetting {
		let restoreWindows: RestoreWindowsSetting;
		if (this.lifecycleMainService.wasRestarted) {
			restoreWindows = 'all'; // always reopen all windows when an update was applied
		} else {
			const windowConfig = this.configurationService.getValue<IWindowSettings | undefined>('window');
			restoreWindows = windowConfig?.restoreWindows || 'all'; // by default restore all windows

			if (!['preserve', 'all', 'folders', 'one', 'none'].includes(restoreWindows)) {
				restoreWindows = 'all'; // by default restore all windows
			}
		}

		return restoreWindows;
	}