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

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

		if (e.leftButton) {
			e.preventDefault();
			this._sliderMouseDown(e, () => { /*nothing to do*/ });
		}
	}