Display sidebar ad when scrolled

There is a JS bug around ad display when linked directly
to an anchor somewhat down the page.
By default, we display an ad in the sidebar if it would be
visible without scrolling the sidebar (who does that?)
and put it in the footer otherwise.
The math was slightly off on whether the bottom of the sidebar
would be visible if the window was already scrolled.

Author: davidfischer

readthedocs/core/static-src/core/js/doc-embed/sponsorship.js

@@ -56,7 +56,7 @@ function create_ad_placement() {
         // If this is off screen, instead create an ad in the footer
         element = $("<div />").appendTo(selector);
         offset = element.offset();
-        if (!offset || offset.top > $(window).height()) {
+        if (!offset || (offset.top - $(window).scrollTop()) > $(window).height()) {
             if (rtd.is_rtd_like_theme()) {
                 selector = $('<div />').insertAfter('footer hr');
                 class_name = 'ethical-rtd';