src/vs/workbench/browser/layout.ts/Layout/updateWindowBorder
class Layout:	private updateWindowBorder(skipLayout: boolean = false) {
		if (isWeb || getTitleBarStyle(this.configurationService) !== 'custom') {
			return;
		}

		const theme = this.themeService.getColorTheme();

		const activeBorder = theme.getColor(WINDOW_ACTIVE_BORDER);
		const inactiveBorder = theme.getColor(WINDOW_INACTIVE_BORDER);

		let windowBorder = false;
		if (!this.state.fullscreen && !this.state.maximized && (activeBorder || inactiveBorder)) {
			windowBorder = true;

			// If the inactive color is missing, fallback to the active one
			const borderColor = this.state.hasFocus ? activeBorder : inactiveBorder ?? activeBorder;
			this.container.style.setProperty('--window-border-color', borderColor?.toString() ?? 'transparent');
		}

		if (windowBorder === this.state.windowBorder) {
			return;
		}

		this.state.windowBorder = windowBorder;

		this.container.classList.toggle(Classes.WINDOW_BORDER, windowBorder);

		if (!skipLayout) {
			this.layout();
		}
	}