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.
Ensure that we never round the canvas dimensions above `maxCanvasPixels`
by rounding them to the preceeding multiple of the display ratio rather
than the succeeding one.
This commit introduces a test to ensure that:
- When zooming below the maxCanvasPixels limit, the canvas is rendered
with the correct size (the two sides are multiplied by the zoom factor).
- When zooming above the maxCanvasPixels limit, the canvas size is capped.
This parameter allows defining which point should remain
fixed while scaling the document. It can be used, for example,
to implement "zoom around the cursor" or "zoom around
pinch center".
The logic was previously implemented in `web/app.js`, but
moving it to the viewer scaling utilities themselves makes it
easier to implement similar zooming functionalities in
other embedders.
This commit adds a test for 0603d1ac1843bc4098d74382beda6cc511350ccd.
Before the fix the `pagerendered` events would be fired just 2-3
milliseconds after the call to `increaseScale`/`decreaseScale`.
