When testing the new `PDFHistory` implementation in practice, I felt that the current value of `UPDATE_VIEWAREA_TIMEOUT` is too large to be truly useful.
The purpose of the timeout is to attempt to address (the PDF.js part of) https://bugzilla.mozilla.org/show_bug.cgi?id=1153393, and it's currently fairly easy for the user e.g. close the browser before the timeout had a change to finish.
Obviously, the timeout is a best-effort solution, but with the current value of `UPDATE_VIEWAREA_TIMEOUT` it's not as useful as one would want.
Please note that lowering it shouldn't be a problem, since it still prevents the browser history from updating at *every* 'updateviewarea' event or during (quick) scrolling, which is all that's really needed to not impact the UX negatively.
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Furthermore, with this lower timeout, we can also simplify the part of the 'popstate' event handler that attempted to update the browser history with the current position before moving back. In most cases, the current position will now already exist in the history, and this *greatly* decreases the complexity of this code path.
The main impetus for this change though, is that I unfortunately found that given the asynchronous nature of updating the browser history, there is some *edge* cases where that code could cause history corruption.
In practice, the user could thus get "stuck" at a particular history entry and not be able to move back. I haven't got any reliable STR for this, since it's so difficult to trigger, but it involved navigating around in a document such that a number of destinations are added to the browser history and then changing the rotation before going back/forward in the history.
Rather that attempting to patch this code, and making it even more difficult to understand than it already is or adding more asynchronous behaviour, by far the easiest solution is to remove it and simply rely on the (lowered) `UPDATE_VIEWAREA_TIMEOUT` instead.
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