*This makes the same kind of changes as in the previous patch, but for the pageNumber-loadingIcon in the main toolbar.*
To display the pageNumber-loadingIcon when rendering starts, if the page is the most visible one, we'll utilize the existing "pagerender" event.
To toggle the pageNumber-loadingIcon as the user moves through the document we'll now instead utilize the "pagechanging" event, which should actually be slightly more efficient overall[1]. Note how we'd, in the old code, only consider the most visible page anyway when toggling the pageNumber-loadingIcon.
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[1] Even in a PDF document as relatively short/simple as `tracemonkey.pdf`, scrolling through the entire document can easily trigger the "updateviewarea" event more than a thousand times.
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