This is a precursor to moving the call into a
worker thread to let us use `OffscreenCanvas`. The
current position wouldn't work since we make
transformations to the canvas object after the
getContext call, which isn't allowed for
OffscreenCanvas. Also it isn't allowed to clone or
`transferControlToOffscreen` the canvas after the
`getContext` call.
Instead, we update the visible canvas every 500ms.
With large canvas, updating at 60fps lead to a lot gfx transactions and it can take a lot of time.
For example, with wuppertal_2012.pdf on Windows, displaying it at 150% takes around 14 min !!! without
this patch when it takes only around 14 sec with. Even at 30% it helps to improve the performance
by around 20%.
This restores the behaviour that existed prior to PR 19128, see e.g. 8727a04ae5/web/pdf_page_view.js (L908-L911), since `RenderingCancelledException` should still overwrite any previously seen Error.
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.
This base class contains the generic logic for:
- Creating a canvas and showing when appropriate
- Rendering in the canvas
- Keeping track of the rendering state
