Note how in `PDFPageViewBuffer.resize` we're manually iterating through the visible pages in order to build a Set of the visible page `id`s. By instead moving the building of this Set into the `getVisibleElements` helper function, as part of the existing parsing, this code becomes *ever so slightly* more efficient.
Furthermore, more direct access to the visible page `id`s also come in handy in other parts of the viewer as well.
In the `BaseViewer.isPageVisible` method we no longer need to loop through the visible pages, but can instead directly check if the pageNumber is visible.
In the `PDFRenderingQueue.getHighestPriority` method, when checking for "holes" in the page layout, we can also avoid some unnecessary look-ups this way.
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