Fix infinite scroll example: intersect trigger not firing

[dienhoa]

The infinite scrolling doesn't work. Scrolling to the bottom of the page doesn't trigger intersect once.

According to the Intersection Observer API:

Note that it's possible to have a zero intersection rectangle, which can happen if the intersection is exactly along the boundary between the two or the area of boundingClientRect is zero. This state of the target and root sharing a boundary line is not considered enough to be considered transitioning into an intersecting state.

So it's more reliable to have a div with a real area (by giving it height) to trigger intersect.

@jph00