fix #### Overlay behaviour when body is scrollable

Loading indicators associated to an element are supposed to be cover and be cenetered only in the visible portion of the element.
This worked correctly apart from the case in which the body of the document has scrollbars. In that case, the overlay was not constrained to the viewport.

This commit fixes the issue by always constraining the overley in the viewport.

Also, loading indicators were not being refreshed on window resize. This feature has been added within this commit.