*This is something that occurred to me when reviewing the latest PDF.js update in mozilla-central.*
Currently we duplicate essentially the same code in both the `OutputScale.prototype.limitCanvas` and `PDFPageDetailView.prototype.update` methods, which seems unnecessary, and to avoid that we introduce a new `OutputScale.capPixels` method that is used to compute the maximum canvas pixels.
This way it helps to reduce the overall canvas dimensions and make the rendering faster.
The drawback is that when scrolling, the page can be blurry in waiting for the rendering.
The default value is 200% on desktop and will be 100% for GeckoView.
Currently we lookup the `devicePixelRatio`, with fallback handling, in a number of spots in the code-base.
Rather than duplicating code we can instead add a new static method in the `OutputScale` class, since that one is now exposed in the API.
When scrolling quickly, the constant re-rendering of the detail view
significantly affects rendering performance, causing Firefox to
not render even the _background canvas_, which is just a static canvas
not being re-drawn by JavaScript.
This commit changes the viewer to only render the detail view while
scrolling if its rendering hasn't just been cancelled. This means that:
- when the user is scrolling slowly, we have enough time to render the
detail view before that we need to change its area, so the user always
sees the full screen as high resolution.
- when the user is scrolling quickly, as soon as we have to cancel a
rendering we just give up, and the user will see the lower resolution
canvas. When then the user stops scrolling, we render the detail view
for the new visible area.
When rendering big PDF pages at high zoom levels, we currently fall back
to CSS zoom to avoid rendering canvases with too many pixels. This
causes zoomed in PDF to look blurry, and the text to be potentially
unreadable.
This commit adds support for rendering _part_ of a page (called
`PDFPageDetailView` in the code), so that we can render portion of a
page in a smaller canvas without hiting the maximun canvas size limit.
Specifically, we render an area of that page that is slightly larger
than the area that is visible on the screen (100% larger in each
direction, unless we have to limit it due to the maximum canvas size).
As the user scrolls around the page, we re-render a new area centered
around what is currently visible.
