Render speed-ups and performance investigations.

WIP.md

@@ -433,7 +433,7 @@ From `docs/`:
 - `check.bat` — link check (offline Lychee against `_site/`).
 - `book.bat` — renders the PDF from `_site-pdf/book.html` via `pagedjs-cli` into `_pdf/book.pdf`. Run `build.bat` first to populate `_site-pdf/`.
 
-The HTML whitespace compression that wraps every page's render chain is handled by `_plugins/html-compress.rb` rather than the just-the-docs theme's `vendor/compress.html` Liquid layout — see [_plugins/html-compress.md](docs/_plugins/html-compress.md) for the full writeup. The Liquid layout's per-page cost in the profile was ~2.4s of Liquid filter dispatch (a `split: " " | join: " "` over the outside-of-`<pre>` content, lowering to a per-page Array allocation of every whitespace-delimited token across 837 pages — millions of small `String` objects). The layout is short-circuited via `compress_html.ignore.envs: all` in `_config.yml`; it then outputs a bare `{{ content }}` and the plugin takes over at `:pages, :post_render` / `:documents, :post_render` with `priority :high`, doing the same pre-block-protected whitespace collapse via `content.split(PRE_BLOCK_RE).each { |s| s.split(" ").join(" ") }` in C-implemented Ruby. The `priority :high` annotation places this hook before offlinify and pdfify (both `:normal`) so they see the compressed bytes. Pages whose layout chain doesn't reach `vendor/compress` are gated out via a `:site, :pre_render` precompute that walks `site.layouts[name].data["layout"]` for every layout key and marks the entire compress-reaching chain (default → table_wrappers → vendor/compress) -- jekyll-redirect-from stubs, the SCSS-derived CSS pages, `assets/js/zzzz-search-data.json`, and `book.html` (which uses the minimal `book-combined` layout that has no parent) all stay un-gated and pass through verbatim, matching exactly what the Liquid layout would have processed. Output is byte-identical to the layout-based version: a recursive `diff -rq` of `_site/` against a vendor/compress.html baseline reports zero differences across all ~840 HTML pages, 290 redirect stubs, every CSS / JSON / SVG / image asset. The plugin's correctness depended on two non-obvious details that broke an earlier cut -- the layout-chain walk has to compare against the layout *key* (`"vendor/compress"`) rather than `layout.name` (which carries the `.html` extension), and the per-segment `split(" ").join(" ")` strips trailing whitespace that the Liquid layout's *template* re-adds via its trailing-newline source character, so the plugin captures `content.end_with?("\n")` before the split and re-appends a `\n` after the join. Both regressions surfaced as nonzero `diff -rq` counts during development and are flagged in the plugin's header comment and [_plugins/html-compress.md](docs/_plugins/html-compress.md).
+The HTML whitespace compression that wraps every page's render chain is handled by `_plugins/html-compress.rb` rather than the just-the-docs theme's `vendor/compress.html` Liquid layout — see [_plugins/html-compress.md](docs/_plugins/html-compress.md) for the full writeup. The Liquid layout's per-page cost in the profile was ~2.4s of Liquid filter dispatch (a `split: " " | join: " "` over the outside-of-`<pre>` content, lowering to a per-page Array allocation of every whitespace-delimited token across 837 pages — millions of small `String` objects). The layout is short-circuited via `compress_html.ignore.envs: all` in `_config.yml`; it then outputs a bare `{{ content }}` and the plugin takes over at `:pages, :post_render` / `:documents, :post_render` with `priority :normal`, doing the same pre-block-protected whitespace collapse via `content.split(PRE_BLOCK_RE).each { |s| s.split(" ").join(" ") }` in C-implemented Ruby. The `:normal` priority is the *middle* tier of a three-level convention across the site's `:post_render` hooks: mutators (`book-href-rewrite`) run at `:high`, this cleanup pass at `:normal`, readers (`pdfify`, `offlinify`) at `:low`. The invariant "compress runs after every mutator and before every reader" therefore holds by construction; no downstream plugin has to be whitespace-aware. Pages whose layout chain doesn't reach `vendor/compress` are gated out via a `:site, :pre_render` precompute that walks `site.layouts[name].data["layout"]` for every layout key and marks the entire compress-reaching chain (default → table_wrappers → vendor/compress) -- jekyll-redirect-from stubs, the SCSS-derived CSS pages, and `assets/js/zzzz-search-data.json` all stay un-gated and pass through verbatim. `book.html` (which uses the minimal `book-combined` layout that has no parent) is *also* outside that chain but is explicitly added to the compress-eligible set at the end of the precompute, so the same whitespace collapse runs on it -- saves paged.js's render-time `WhiteSpaceFilter` ~37k DOM mutations (~28k `textContent` overwrites + ~9k `removeChild` calls) at the cost of ~480 ms once per Jekyll build. Output is byte-identical to the layout-based version: a recursive `diff -rq` of `_site/` against a vendor/compress.html baseline reports zero differences across all ~840 HTML pages, 290 redirect stubs, every CSS / JSON / SVG / image asset. The plugin's correctness depended on two non-obvious details that broke an earlier cut -- the layout-chain walk has to compare against the layout *key* (`"vendor/compress"`) rather than `layout.name` (which carries the `.html` extension), and the per-segment `split(" ").join(" ")` strips trailing whitespace that the Liquid layout's *template* re-adds via its trailing-newline source character, so the plugin captures `content.end_with?("\n")` before the split and re-appends a `\n` after the join. Both regressions surfaced as nonzero `diff -rq` counts during development and are flagged in the plugin's header comment and [_plugins/html-compress.md](docs/_plugins/html-compress.md).
 
 ### Profiling the build
 

docs/_plugins/book-href-rewrite.rb

@@ -372,7 +372,11 @@ def self.process(page)
   end
 end
 
-Jekyll::Hooks.register :pages, :post_render do |page|
+# :high so this MUTATOR runs before html-compress (priority :normal).
+# Otherwise the landing-heading strip leaves a double-space run that
+# no downstream pass cleans up. See html-compress.rb's priority
+# convention comment for the full layering.
+Jekyll::Hooks.register :pages, :post_render, priority: :high do |page|
   next unless page.path == "book.html"
   BookHrefRewrite.process(page)
 end

docs/_plugins/html-compress.md

@@ -1,6 +1,6 @@
 # HtmlCompress
 
-`_plugins/html-compress.rb` runs the HTML whitespace compression that wraps every page's render chain — the same job just-the-docs's vendor/compress.html Liquid layout was doing, but in Ruby instead of Liquid filters. Output is byte-identical to the layout-based version (verified by recursive diff of every file in `_site/` against a vendor/compress.html baseline). The Liquid layout is short-circuited to a `{{ content }}` passthrough via `compress_html.ignore.envs: all` in `_config.yml`; the plugin then runs at `:pages, :post_render` / `:documents, :post_render` with `priority :high`, so the compressed bytes are what offlinify and Jekyll's writer see.
+`_plugins/html-compress.rb` runs the HTML whitespace compression that wraps every page's render chain — the same job just-the-docs's vendor/compress.html Liquid layout was doing, but in Ruby instead of Liquid filters. Output is byte-identical to the layout-based version for the 837 vendor/compress-reaching pages (verified by recursive diff of every file in `_site/` against a vendor/compress.html baseline). The Liquid layout is short-circuited to a `{{ content }}` passthrough via `compress_html.ignore.envs: all` in `_config.yml`; the plugin then runs at `:pages, :post_render` / `:documents, :post_render` with `priority :normal` as the *cleanup* step in a three-tier `:high` → `:normal` → `:low` ordering (mutators → compress → readers — see [Hook priority convention](#hook-priority-convention) below). It also picks up one page the original layout didn't process, `book.html`, via an explicit `book-combined` addition to the compress-eligible set — see [book.html inclusion](#bookhtml-inclusion).
 
 This file sits in `_plugins/` for the same reasons as `offlinify.md` and `pdfify.md`: it lives next to the code it documents, and Jekyll's `_plugins/` folder is plugin-only territory, so this Markdown never gets rendered into the public site.
 
@@ -74,7 +74,7 @@ page.md   (layout: default)
         └── vendor/compress.html (no layout)
 ```
 
-Pages that don't use any of these layouts — jekyll-redirect-from stubs, the SCSS-derived CSS pages, `assets/js/zzzz-search-data.json`, `book.html` (which uses the minimal `book-combined` layout that has no parent) — were left untouched by the layout. The plugin has to match that gating, otherwise it would compress files that compress.html doesn't, breaking byte-identity.
+Pages that don't use any of these layouts — jekyll-redirect-from stubs, the SCSS-derived CSS pages, `assets/js/zzzz-search-data.json` — were left untouched by the layout. The plugin has to match that gating, otherwise it would compress files that compress.html doesn't, breaking byte-identity. `book.html` (which uses the minimal `book-combined` layout that has no parent) was originally in this list, but is now explicitly added to the compress-eligible set — see [book.html inclusion](#bookhtml-inclusion).
 
 The gate is precomputed once at `:site, :pre_render`:
 
@@ -114,20 +114,42 @@ Jekyll::Hooks.register :site, :pre_render do |site|
   HtmlCompress.precompute_compress_layouts!(site)
 end
 
-Jekyll::Hooks.register :pages, :post_render, priority: :high do |page|
+Jekyll::Hooks.register :pages, :post_render, priority: :normal do |page|
   next unless page.output.is_a?(String)
   next unless HtmlCompress.compress?(page)
   HtmlCompress.compress!(page.output)
 end
 
-Jekyll::Hooks.register :documents, :post_render, priority: :high do |doc|
+Jekyll::Hooks.register :documents, :post_render, priority: :normal do |doc|
   next unless doc.output.is_a?(String)
   next unless HtmlCompress.compress?(doc)
   HtmlCompress.compress!(doc.output)
 end
 ```
 
-The `priority: :high` is what places the plugin *before* `offlinify.rb` and `pdfify.rb` in the per-page render-hook order — both of those use the default `:normal` priority and rely on reading the final compressed `page.output`. Jekyll runs `:post_render` hooks in descending priority, so `:high` (30) fires before `:normal` (20). Without the priority annotation the order would be insertion-order across all `.rb` files in `_plugins/`, which is not a stable contract.
+## Hook priority convention
+
+The `priority: :normal` is the middle tier of a three-level ordering for `:pages, :post_render` and `:documents, :post_render` hooks across the plugin set. Jekyll runs hooks in descending priority (`:high` (30) → `:normal` (20) → `:low` (10)), and the three tiers carry distinct roles:
+
+| Tier | Role | Plugins |
+| --- | --- | --- |
+| `:high` (30) | **Mutators.** Modify `page.output` so the final bytes reflect this pass. | `book-href-rewrite` (chapter href rewrites + landing-heading strip on `book.html`). |
+| `:normal` (20) | **Compress.** The cleanup pass. Sandwiched between mutators and readers so any whitespace runs left behind by a mutator's `gsub` get collapsed before any reader captures the bytes. | `html-compress` (this plugin). |
+| `:low` (10) | **Readers.** Snapshot or consume `page.output` after the cleanup pass. | `pdfify` (captures `book.html` for the PDF pipeline), `offlinify` (per-page href / src rewrites + write to `_site-offline/`). |
+
+The layering was originally implicit: the plugin sat at `:high` next to no other priority-annotated `:post_render` hooks. That worked until `book-href-rewrite` joined the set at default `:normal`. Its landing-heading strip ran *after* compress, removing `<h2>` blocks but leaving the (already-collapsed) single-space runs on either side adjacent — producing literal `>  <` blobs in three chapter openings that paged.js's WhiteSpaceFilter then had to handle at render time. Promoting `book-href-rewrite` to `:high` and demoting compress to `:normal` makes the invariant "compress is the last cleanup step among mutators" hold by construction; demoting the readers to `:low` makes "readers see the final compressed output" hold by construction. Future plugins choose their tier by their role and the ordering composes automatically.
+
+The full priority story is documented as a comment block above the `Jekyll::Hooks.register` calls in [`html-compress.rb`](html-compress.rb); each of the four affected plugins (this one, `book-href-rewrite`, `pdfify`, `offlinify`) carries a one-line note pointing back to that block.
+
+## book.html inclusion
+
+The layout-chain walk above only marks layouts that reach `vendor/compress`. `book.html` uses the minimal `book-combined` layout, which has no parent, so the walk never reaches it and the page was originally skipped (matching the layout's behaviour). After investigation of paged.js's per-render `WhiteSpaceFilter` work (see [`perf/README.md`](../../perf/README.md)) showed it doing ~37k DOM mutations at render time to handle whitespace text nodes that *would* have been collapsed if the page had been compressed at Jekyll build time, the precompute was extended to mark `book-combined` explicitly:
+
+```ruby
+@compress_layouts << "book-combined" if site.layouts.key?("book-combined")
+```
+
+at the end of `precompute_compress_layouts!`. Output: `book.html` now passes through `compress!` once per build (~480 ms of additional `String#split` work on the ~5.5 MB document), saving roughly the same wall-clock at paged.js render time (~28k `textContent` overwrites + ~9k `removeChild` calls eliminated). Net is approximately wall-clock-neutral for full builds, and a small net win for incremental Jekyll workflows that skip the PDF (`also_build_pdf: false`) — the compress cost is paid once per Jekyll build, the render saving is paid every PDF build, and decoupling the two is the structural improvement.
 
 ## Verification
 
@@ -157,6 +179,6 @@ In source order in [`html-compress.rb`](html-compress.rb):
 
 - `precompute_compress_layouts!(site)` — `:site, :pre_render` entry. Walks every layout chain via `data["layout"]`, marks each layout on the path as compress-ending the moment the walk hits `vendor/compress`. Idempotent; the resulting `@compress_layouts` set persists across builds in `jekyll serve` and gets rebuilt fresh each `:pre_render`.
 
-- `compress?(page)` — gate check. Returns `true` when the page's `data["layout"]` is in `@compress_layouts`. Pages without a layout (jekyll-redirect-from stubs, SCSS-derived CSS, JSON-via-page-rendering, `book.html` via `book-combined`) return `false` and skip the compression entirely.
+- `compress?(page)` — gate check. Returns `true` when the page's `data["layout"]` is in `@compress_layouts`. Pages without a layout (jekyll-redirect-from stubs, SCSS-derived CSS, JSON-via-page-rendering) return `false` and skip the compression entirely. `book.html` (which uses `book-combined`, a minimal layout with no parent) used to land here too; it is now explicitly added to the set by `precompute_compress_layouts!` — see [book.html inclusion](#bookhtml-inclusion).
 
 - `compress!(content)` — the actual compression, in place. Captures the trailing-newline state, splits by `PRE_BLOCK_RE` with the capture group so pre bodies are preserved in the result array, runs `split(" ").join(" ")` on every outside-of-pre segment, joins, restores the trailing newline if needed, then mutates the input string via `String#replace`. The `replace` is what lets us hand back the same string object the caller passed in — Jekyll's writer reads `page.output` after `:post_render`, so in-place mutation is the cheapest way to update what gets written.

docs/_plugins/html-compress.rb

@@ -72,6 +72,12 @@ def self.precompute_compress_layouts!(site)
         cur_name = cur ? cur.data["layout"] : nil
       end
     end
+    # book-combined is a minimal layout with no parent, so the walk
+    # above doesn't reach it. Compressing its only consumer (book.html)
+    # at Jekyll time saves paged.js's WhiteSpaceFilter ~37k DOM
+    # mutations and ~300-400 ms once per render -- see
+    # perf/README.md "WhiteSpaceFilter that wasn't" section.
+    @compress_layouts << "book-combined" if site.layouts.key?("book-combined")
   end
 
   # True when `page` (or document) uses a layout chain ending in
@@ -117,16 +123,43 @@ def self.compress!(content)
   HtmlCompress.precompute_compress_layouts!(site)
 end
 
-# Run before offlinify (default :normal priority) so the offline-tree
-# rewrites see the compressed page.output, and before Jekyll's
-# `:site, :post_write` writes _site/ for the same reason.
-Jekyll::Hooks.register :pages, :post_render, priority: :high do |page|
+# Priority convention for :pages, :post_render hooks in this site:
+#
+#   :high   = MUTATORS. Plugins that modify page.output. Run first so
+#             their mutations are visible to compress and downstream
+#             readers. Examples: book-href-rewrite (landing heading
+#             strip + in-book href rewrites).
+#
+#   :normal = COMPRESS. This plugin. The cleanup pass, sandwiched
+#             between mutators and readers so any whitespace runs left
+#             behind by a mutator's gsub get collapsed before anyone
+#             reads the final bytes.
+#
+#   :low    = READERS. Plugins that snapshot or consume page.output
+#             after all mutations and the compress pass. Run last so
+#             they see final output. Examples: pdfify (captures
+#             book.html for the PDF pipeline), offlinify (rewrites
+#             root-absolute hrefs and writes to _site-offline/).
+#
+# Without this layering, a mutator running after compress leaves
+# adjacent whitespace runs that no downstream pass collapses; a
+# reader running before compress captures uncompressed bytes. Both
+# regressions surfaced when book-href-rewrite (default :normal) ran
+# after html-compress (originally :high) -- its 3 landing-heading
+# strips left double-space artifacts that paged.js's WhiteSpaceFilter
+# had to handle at render time.
+#
+# Offlinify also runs at :site, :post_write (a later phase entirely),
+# where it always sees the final compressed bytes regardless of
+# per-page priority. The :low designation here governs its per-page
+# capture hook specifically.
+Jekyll::Hooks.register :pages, :post_render, priority: :normal do |page|
   next unless page.output.is_a?(String)
   next unless HtmlCompress.compress?(page)
   HtmlCompress.compress!(page.output)
 end
 
-Jekyll::Hooks.register :documents, :post_render, priority: :high do |doc|
+Jekyll::Hooks.register :documents, :post_render, priority: :normal do |doc|
   next unless doc.output.is_a?(String)
   next unless HtmlCompress.compress?(doc)
   HtmlCompress.compress!(doc.output)

docs/_plugins/offlinify.rb

@@ -1443,11 +1443,13 @@ def self.decode(path)
   Offlinify.setup(site)
 end
 
-Jekyll::Hooks.register :pages, :post_render do |page|
+# :low so these READERS see page.output after html-compress (:normal)
+# has run. See html-compress.rb's priority convention.
+Jekyll::Hooks.register :pages, :post_render, priority: :low do |page|
   Offlinify.process_page(page)
 end
 
-Jekyll::Hooks.register :documents, :post_render do |doc|
+Jekyll::Hooks.register :documents, :post_render, priority: :low do |doc|
   Offlinify.process_page(doc)
 end
 

docs/_plugins/pdfify.rb

@@ -287,7 +287,9 @@ def self.copy_file(src, dst)
   Pdfify.setup(site)
 end
 
-Jekyll::Hooks.register :pages, :post_render do |page|
+# :low so this READER captures page.output after html-compress
+# (:normal) has run. See html-compress.rb's priority convention.
+Jekyll::Hooks.register :pages, :post_render, priority: :low do |page|
   Pdfify.maybe_capture(page)
 end