*This fixes a regression from PR 6192.*
Under some circumstances, the `resize` event handler in `viewer.js` is fired before the scale has been set. This can lead to PDF documents being rendered at the wrong zoom level when they are opened.
It seems that a way to reliably trigger this is to, using the Firefox addon, open a PDF file that triggers the `fallback` bar, in a new background tab (i.e. middle clicking on a link, or use the context menu).
Prior to PR 6192, we checked the selected option in the `scaleSelect` dropdown instead. Since `pageAutoOption` is selected by default in `viewer.html`, this should explain why the issue wasn't visible previously.
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