QQuickItem::childAt does not necessarily return a visible item

QQuickItem::childAt is described to return the first visible item at the specified location.

It returns the first applicable child item in childItems regardless of paint order.  This means there are cases where it can return a child item that is not currently visible.

The function should probably return the first applicable child out of the sortedChildItems list instead.  This would ensure the returned item is the best possible match.

This bug was found in 5.5.1, but it looks like the source code is still the same in later versions (most recent in git repo as of 09/25/2018)