Does not 2-finger scroll when mouse is over the left most 50px of div B. Found in Chrome 49.0.2623.75+. Works in Chrome 48.0.2564.103-.

The z-index, border-radius and overflow rules on div A are essential. If you remove either of them things will work again.

The area of non-functional scroll behavior "b" is in direct proportion to distance "a" the left position of div B. If you change that to 25px the non-functional area will be reduced. If you change it to 0px the problem will not be apparent.