[andiwand]

Refs #764.

Summary

This is level 3 for a pdf: option B of docs/design/lazy-loading.md decision 7 (#1256). With HtmlConfig::lazy_loading, the combined document renders its first page only. Each later page is an empty page box that loads pdf-pages/<n>.js, a render of that page alone.

• What a script brings: a <style> with its page's rules, and the content of its page box. The placeholder already is the page box, with its size, page:, id="pN" and data-odr-page. So odr.chunks.fill works as it does for text, and an internal #pN link still lands.
• Classes:
  • A page rendered alone suffixes its class, face and SVG def names with its number (StyleRegistry::set_suffix, SubstituteFontFaces): x0_12, odr-s1_12.
  • Every script states the .fvN rule of its fonts again, and later in the document. So in a document that loads its pages, the registry rules state their class three times (Rank::replaces_inline). Without that, a later page's .fvN{color:#000} turned an earlier page's blue link text black. A screenshot comparison caught this.
• Fonts:
  • The dual layer re-encodes a font to the PUA range only, so one file serves every page.
  • The document lists the fonts that the pages and their forms state in their resources, before page 1 renders. It numbers each font for the whole service and links it as pdf-fonts/<n>.ttf, which bring_offline writes and the server serves.
  • A font that no resources state stays inline in its page.
• Images: a script carries its images inline, because nobody writes the resources of a script.
• The single layer keeps every page in the document, because it re-encodes a font for the runs it meets.
• With the default config, the output does not change: a full regen differs from the pinned references only in the long-stale editing.js. So no pins move.

Measured on Core_v5.1.pdf (2,985 pages):

	before	at level 3
document, over HTTP	5.4 s to first byte	0.5 s
page script / font file, over HTTP		3–6 ms / <1 ms
document.html	127 MB	1.4 MB + 2,984 scripts + 84 fonts (1.8 MB)
core memory to write the document	761 MB	138 MB
first frame from file://	1.8 s	92 ms

Screenshots at 0, 30%, 50% and 90% of the document match the full render to the pixel, from file:// and over HTTP.

Test

• html.a_pdf_loads_its_later_pages covers opendocument-app-website.pdf (8 pages):
  • the placeholder of page 2 and the resource of page 8
  • fonts linked as files, and none inline
  • the script of page 2: its key, its suffixed names, and no font data
  • a font file served
  • the default config with no placeholder
• All 2,535 tests pass.

Review fixes

• clang-tidy 18 refused the loop that collects the page sizes and the empty class callback that took a string by value. The loop now reserves, and the callback takes a reference.
• The CHANGELOG entry says that each page renders when the reader comes near it, and uses "document" for the html file.
• The design doc lists the single layer at level 3 as an open question, with its two ways: fonts inline in each script, or one vote over the whole document.

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