Fixes sidebar fade-in for short pages
Fixes sidebar fade-in for short pages

[#48685567] The Upstatement code faded in the sidebar once the page had been scrolled halfway to the content top (i.e. halfway through the header and introduction), but this meant if the page was too short to scroll that far (or at all) the sidebar tab never appeared. Now we are just fading the tab in at page load, regardless of scrolling. 

(It might be possible to evaluate this fade-in depending on how tall the page is, using Upstatement's strategy if the page is tall enough but falling back to this if it isn't.)
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