src/vs/base/browser/ui/scrollbar/scrollbarState.ts/ScrollbarState/getDesiredScrollPositionFromDelta
class ScrollbarState:	public getDesiredScrollPositionFromDelta(delta: number): number {
		if (!this._computedIsNeeded) {
			// no need for a slider
			return 0;
		}

		const desiredSliderPosition = this._computedSliderPosition + delta;
		return Math.round(desiredSliderPosition / this._computedSliderRatio);
	}