We ‘shim’ the sticky element so that its space in the page is reserved
while it is sticky.
This means the shim have have the same height as the sticky element.
It’s height was not getting set because of a missing semicolon in the
shim element’s `style` attribute.
Position of elements are normally checked when you
scroll but we also need it to check when the page
loads.
Re-calculate element positions if window resizes.
Adds a flag to mark if all elements have a height
which will not change as their contents have
loaded.
