src/vs/workbench/browser/parts/sidebar/sidebarPart.ts/SidebarPart/preferredWidth
class SidebarPart:	get preferredWidth(): number | undefined {
		const viewlet = this.getActiveViewlet();

		if (!viewlet) {
			return;
		}

		const width = viewlet.getOptimalWidth();
		if (typeof width !== 'number') {
			return;
		}

		return Math.max(width, 300);
	}