src/vs/workbench/browser/parts/panel/panelPart.ts/BasePanelPart/preferredWidth
class BasePanelPart:	get preferredWidth(): number | undefined {
		const activeComposite = this.getActivePaneComposite();

		if (!activeComposite) {
			return;
		}

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

		return Math.max(width, 300);
	}