src/vs/base/browser/ui/scrollbar/abstractScrollbar.ts/AbstractScrollbar/_onPointerDown
class AbstractScrollbar:	private _onPointerDown(e: PointerEvent): void {
		let offsetX: number;
		let offsetY: number;
		if (e.target === this.domNode.domNode && typeof e.offsetX === 'number' && typeof e.offsetY === 'number') {
			offsetX = e.offsetX;
			offsetY = e.offsetY;
		} else {
			const domNodePosition = dom.getDomNodePagePosition(this.domNode.domNode);
			offsetX = e.pageX - domNodePosition.left;
			offsetY = e.pageY - domNodePosition.top;
		}

		const offset = this._pointerDownRelativePosition(offsetX, offsetY);
		this._setDesiredScrollPositionNow(
			this._scrollByPage
				? this._scrollbarState.getDesiredScrollPositionFromOffsetPaged(offset)
				: this._scrollbarState.getDesiredScrollPositionFromOffset(offset)
		);

		if (e.button === 0) {
			// left button
			e.preventDefault();
			this._sliderPointerDown(e);
		}
	}