*This fixes a regression from commit c9a0955c9c, i.e. PR 7738.*
Currently if you quickly rotate a document at least *twice*,[1] such that rendering of a page hasn't finished for the first rotation before the last rotation is triggered, the `cssTransform` method can fail to update the page correctly leading to it looking temporarily distorted.
The reason why things break is that previously we stored the `viewport` on the canvas DOM element, meaning that when it was accessed in `cssTransform` is was guaranteed to point to the `viewport` of the `zoomLayer` canvas.
Generally you want to avoid storing data on DOM elements this way, and during the `PDFPageView` refactoring needed to support SVG rendering, the previous `viewport` was instead stored directly on `PDFPageView`.
However, the problem is first of all that the `paintedViewport` only stores the *last* `viewport` computed, and second of all that there're no guarantees that it actually applies to the current `zoomLayer` canvas.
If a document is rotated slowly enough that rendering finishes *before* the next rotation then this problem doesn't exist, but for sufficiently quick rotations rendering will be cancelled at least once and the `paintedViewport` could thus be bogus.
The solution for the above problems is to ensure that we track the correct `viewport` for each DOM element (canvas or svg),[2] which seemed easist to do with a `WeakMap`.[3]
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[1] I'm able to reproduce this using the `tracemonkey` file, but please note that for pages with few operations, i.e. that render very quickly, the effect may be hard to spot.
[2] One other possible solution that I briefly considered, was to wait until rendering finished before storing the current `viewport`. However, that would have caused issues with rotating a page before the *first* rendering operation had finished.
[3] This regression took me way longer to both figure out, and fix, than I'd like to admit :-)
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