src/vs/base/browser/ui/splitview/paneview.ts/Pane/updateHeader
class Pane:	protected updateHeader(): void {
		const expanded = !this.headerVisible || this.isExpanded();

		this.header.style.lineHeight = `${this.headerSize}px`;
		this.header.classList.toggle('hidden', !this.headerVisible);
		this.header.classList.toggle('expanded', expanded);
		this.header.setAttribute('aria-expanded', String(expanded));

		this.header.style.color = this.styles.headerForeground ? this.styles.headerForeground.toString() : '';
		this.header.style.backgroundColor = this.styles.headerBackground ? this.styles.headerBackground.toString() : '';
		this.header.style.borderTop = this.styles.headerBorder && this.orientation === Orientation.VERTICAL ? `1px solid ${this.styles.headerBorder}` : '';
		this._dropBackground = this.styles.dropBackground;
		this.element.style.borderLeft = this.styles.leftBorder && this.orientation === Orientation.HORIZONTAL ? `1px solid ${this.styles.leftBorder}` : '';
	}