src/vs/base/browser/ui/sash/sash.ts/Sash/onMouseLeave
class Sash:	private static onMouseLeave(sash: Sash, fromLinkedSash: boolean = false): void {
		sash.hoverDelayer.cancel();
		sash.el.classList.remove('hover');

		if (!fromLinkedSash && sash.linkedSash) {
			Sash.onMouseLeave(sash.linkedSash, true);
		}
	}