This patch adds right-click support for images in the PDF, allowing
users to download them. To minimize memory consumption, we:
- Do not store the images separately, and instead crop them out of the
PDF page canvas
- Only extract the images when needed (i.e. when the user right-clicks
on them), rather than eagery having all of them available.
To do so, we layer one empty 0x0 canvas per image, stretched to cover
the whole image, and only populate its contents on right click.
These images need to be inside the text layer: they cannot be _behind_
it, otherwise they would be covered by the text layer's container and
not be clickable, and they cannot be in front of it, otherwise they
would make the text spans unselectable.
This feature is managed by a new preference, `imagesRightClickMinSize`:
- when it's set to `-1`, right-click support is disabled
- when set to `0`, all images are available for right click
- when set to a positive integer, only images whose width and height are
greater than or equal to that value (in the PDF page frame of
reference) are available for right click.
This features is disabled by default outside of MOZCENTRAL, as it
significantly degrades the text selection experience in non-Firefox
browsers.
This behaviour comes from the initial pdf.js commit but is wrong and
doesn't match other PDF readers like muPDF or pdfium.
From PDF Spec 7.3.3:
A PDF writer shall not use the PostScript language syntax for numbers with non-decimal radices (such
as 16#FFFE) or in exponential format (such as 6.02E23).
We tried to lookup the font metrics using the font name as-is, which didn't work since the PDF file in question has non-embedded fonts with names that include commas.
Hence the font names need to be normalized here as well, similar to elsewhere in the font code.
For now, `BrotliDecode` hasn't been specified but it should be in a
close future.
So when it's possible we use the native `DecompressionStream` API
with "brotli" as argument.
If that fails or if we've to decompress in a sync context, we fallback
to `BrotliStream` which a pure js implementation (see README in external/brotli).
The bug was supposed to be fixed by #20471 but here there are some annotations in the pdf.
When those annotations are added to the DOM, the struct tree has to be rendered but without
the text layer (because of asynchronicity).
So this patch is making sure that the modifications in the text layer are done once the
layer is rendered.
For now it's just possible to create a single pdf in selecting some pages in different pdf sources.
The merge is for now pretty basic (it's why it's still a WIP) none of these data are merged for now:
- the struct trees
- the page labels
- the outlines
- named destinations
For there are 2 new ref tests where some new pdfs are created: one with some extracted pages and an other
one (encrypted) which is just rewritten.
The ref images are generated from the original pdfs in selecting the page we want and the new images are
taken from the generated pdfs.
It'll help to make math equations "visible" for screen readers.
MS Office has a specific way to add some MathML code to struc tree leaf
and this patch handles it.
