_getVerticalDragRatio(panY) {
    var _this$pswp$currSlide$, _this$pswp$currSlide;

    return (panY - ((_this$pswp$currSlide$ = (_this$pswp$currSlide = this.pswp.currSlide) === null || _this$pswp$currSlide === void 0 ? void 0 : _this$pswp$currSlide.bounds.center.y) !== null && _this$pswp$currSlide$ !== void 0 ? _this$pswp$currSlide$ : 0)) / (this.pswp.viewportSize.y / 3);
  }
Relation between pan Y position and third of viewport height.

When we are at initial position (center bounds) - the ratio is 0,
if position is shifted upwards - the ratio is negative,
if position is shifted downwards - the ratio is positive.

@private
@param {number} panY The current pan Y position.
@returns {number}