As requested in #5178, this change debounces the scroll events.
The reason for doing so is that browsers can event-storm especially on
scroll, communicating hundreds of subpixel changes.
The main reason for this resulting in poor performance is that on each
scroll event `scrollTop` was queried, which forces layouting.
This change will use `requestAnimationFrame` to make sure the browser can
allocate enough time to other tasks. The delay is however quite small, thus
the reduction in executions is less noticeable. Modern browsers however utilize
`requestAnimationFrame` to smoothen out rendering.
Since the "Previous/Next Page" buttons are disabled when the first/last page is active, I think that for the sake of consistency the "Go to First/Last Page" should behave the same way.
Currently the File size field in the Document Properties dialog isn't set until `PDFView.pdfDocument.getDownloadInfo()` is resolved. If the Document Properties dialog is opened while a PDF file is loading with range requests, this leads to the less desirable situation where all fields *except* File size are available.
In cases where `PDFView.open()` is called with the `args` parameter defined, and `args` contains the property `length`, we actually know the File size when the PDF file begins to load. (This is usually the case when ranged loading is used in the Firefox addon/built-in version.)
Hence we can use `args.length` to set the File size immediately when `PDFView.open()` is called, resulting in a better user experience.
QtWebKit does not support file:-URLs in Web Workers.
To solve this issue, I'm loading file:-URLs in the main thread
for generic builds. file:-URLs load relatively quick, so there
is no need for sophisticated load progress notifications.
The absence of this property caused the following error in PDFView.download:
Cannot read property 'split' of undefined
This property was unset when the PDF was loaded through a typed array instead
of a URL. With the fix, the file name will default to "document.pdf", as
defined by getPDFFileNameFromURL in web/ui_utils.js
Currently (at least in Firefox) when the HOME/END keys are pressed, this will trigger unnecessary rendering of pages that lay between the current page and the first/last page. Avoid this by going straight to the first/last page instead.
