src/vs/base/browser/ui/sash/sash.ts/Sash/onMouseEnter
class Sash:	private static onMouseEnter(sash: Sash, fromLinkedSash: boolean = false): void {
		if (sash.el.classList.contains('active')) {
			sash.hoverDelayer.cancel();
			sash.el.classList.add('hover');
		} else {
			sash.hoverDelayer.trigger(() => sash.el.classList.add('hover'), sash.hoverDelay).then(undefined, () => { });
		}

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