Improve offset calculation
Improve offset calculation

[getBoundingClientRect](https://developer.mozilla.org/en-US/docs/Web/API/Element/getBoundingClientRect) can directly determine the actual height of an element and is [supported in all modern browsers](https://caniuse.com/#feat=getboundingclientrect).
Also the fixed header element needs to be looked up only once at initialization 
(if the element somehow changes, which should be quite a rare use/design case, this can be then adjusted in the example code).
cferdinandi/gumshoe,cferdinandi/gumshoe