Sidenotes: emit the section.footnotes fallback the CSS expects

The filter consumes every Pandoc Note, so the "standard Pandoc-
generated section.footnotes" its doc claimed as the no-JS fallback
never existed — below 1500px with JS disabled, footnote content was
simply invisible (AUDIT §2.3). The filter now collects consumed notes
and appends the section itself: letter labels, jump targets for the
in-text refs (which now point at the visible fallback item), and
doc-backlink returns. sidenotes.js pairs ref/note by element id and
preventDefaults clicks, so behavior with JS is unchanged.

Verified in output: per-page item count matches inline sidenote count;
refs target #fn-<label>; backlinks target #snref-<label>.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

build/Filters/Sidenotes.hs

@@ -4,12 +4,23 @@
 --
 --   Each footnote becomes:
 --   * A @<sup class="sidenote-ref">@ anchor in the body text.
---   * An @<aside class="sidenote">@ immediately following it, containing
+--   * A @<span class="sidenote">@ immediately following it, containing
 --     the rendered note content.
 --
---   On wide viewports, sidenotes.css floats asides into the right margin.
---   On narrow viewports they are hidden; the standard Pandoc-generated
---   @<section class="footnotes">@ at the document end serves as fallback.
+--   Additionally, every consumed note is re-emitted in a
+--   @<section class="footnotes">@ appended at the document end. The
+--   filter swallows Pandoc's own @Note@ inlines, so Pandoc's writer
+--   never produces that section itself — without this re-emission,
+--   narrow viewports with JavaScript disabled (where sidenotes.css
+--   hides @.sidenote@ and sidenotes.js's bottom sheet never runs)
+--   would lose footnote content entirely.
+--
+--   On wide viewports, sidenotes.css floats the spans into the right
+--   margin and hides @section.footnotes@; on narrow viewports the
+--   spans are hidden and the section is shown. The in-text anchor
+--   targets the footnotes item (the only target visible on narrow
+--   no-JS viewports); sidenotes.js intercepts clicks and pairs
+--   ref\/note by element id, so the href is purely the no-JS path.
 module Filters.Sidenotes (apply) where
 
 import           Control.Monad.State.Strict
@@ -23,17 +34,53 @@ import           Text.Pandoc.Options        (WriterOptions (..),
 import           Text.Pandoc.Walk           (walkM)
 import           Text.Pandoc.Writers.HTML   (writeHtml5String)
 
--- | Transform all @Note@ inlines in the document to inline sidenote HTML.
-apply :: Pandoc -> Pandoc
-apply doc = evalState (walkM convertNote doc) (1 :: Int)
+-- | Accumulator: next label counter plus collected notes
+--   (newest-first; reversed before rendering the fallback section).
+type NoteState = (Int, [(Text, [Block])])
 
-convertNote :: Inline -> State Int Inline
+-- | Transform all @Note@ inlines in the document to inline sidenote
+--   HTML, and append the collected notes as a @section.footnotes@
+--   fallback block.
+apply :: Pandoc -> Pandoc
+apply doc =
+    let (Pandoc m blocks, (_, collected)) =
+            runState (walkM convertNote doc) (1, [])
+        notes = reverse collected
+    in  Pandoc m $
+            if null notes
+                then blocks
+                else blocks ++ [footnotesSection notes]
+
+convertNote :: Inline -> State NoteState Inline
 convertNote (Note blocks) = do
-    n <- get
-    put (n + 1)
+    (n, acc) <- get
+    put (n + 1, (toLabel n, blocks) : acc)
     return $ RawInline "html" (renderNote n blocks)
 convertNote x = return x
 
+-- | The end-of-document fallback list. Letter labels are rendered
+--   explicitly (an @<ol>@'s automatic numbering would disagree with
+--   the in-text letters), so the list itself is unstyled.
+footnotesSection :: [(Text, [Block])] -> Block
+footnotesSection notes = RawBlock "html" $ T.concat $
+    [ "<section class=\"footnotes\" role=\"doc-endnotes\">"
+    , "<ol class=\"footnotes-list\">"
+    ]
+    ++ map item notes ++
+    [ "</ol>"
+    , "</section>"
+    ]
+  where
+    item (lbl, blocks) = T.concat
+        [ "<li id=\"fn-", lbl, "\" class=\"footnote-item\">"
+        , "<span class=\"footnote-label\" aria-hidden=\"true\">", lbl, "</span>"
+        , blocksToHtml blocks
+        , "<a href=\"#snref-", lbl
+        , "\" class=\"footnote-back\" role=\"doc-backlink\""
+        , " aria-label=\"Back to reference ", lbl, "\">\x21a9\xfe0e</a>"
+        , "</li>"
+        ]
+
 -- | Convert a 1-based counter to a letter label using base-26 expansion
 --   (Excel-column style): 1→a, 2→b, … 26→z, 27→aa, 28→ab, … 52→az,
 --   53→ba, … 702→zz, 703→aaa.  Guarantees a unique label per counter so
@@ -54,8 +101,14 @@ renderNote n blocks =
     let inner = blocksToInlineHtml blocks
         lbl   = toLabel n
     in T.concat
+        -- href targets the footnotes-section item: on narrow no-JS
+        -- viewports that is the only visible rendering of the note
+        -- (the adjacent .sidenote span is display:none there, and on
+        -- wide viewports the note is already visible in the margin).
+        -- sidenotes.js pairs ref/note by id and preventDefaults the
+        -- click, so the href only ever navigates without JS.
         [ "<sup class=\"sidenote-ref\" id=\"snref-", lbl, "\">"
-        ,   "<a href=\"#sn-", lbl, "\">", lbl, "</a>"
+        ,   "<a href=\"#fn-", lbl, "\">", lbl, "</a>"
         , "</sup>"
         , "<span class=\"sidenote\" id=\"sn-", lbl, "\">"
         ,   "<sup class=\"sidenote-num\">", lbl, "</sup>\x00a0"

static/css/sidenotes.css

@@ -16,8 +16,10 @@
    For an inline <span> inside a <p>, this is roughly the line containing
    the sidenote reference, giving correct vertical alignment without JS.
 
-   On narrow viewports the <span> is hidden and the Pandoc-generated
-   <section class="footnotes"> at document end is shown instead.
+   On narrow viewports the <span> is hidden and the
+   <section class="footnotes"> the Sidenotes filter appends at document
+   end is shown instead (Pandoc's own footnote section never exists —
+   the filter consumes every Note, and re-emits this fallback itself).
 */
 
 /* ============================================================
@@ -137,22 +139,54 @@
 
 
 /* ============================================================
-   FOOTNOTE REFERENCES — shown on narrow viewports alongside
-   section.footnotes
+   FOOTNOTES FALLBACK LIST — the section the Sidenotes filter
+   appends at document end; visible on narrow viewports only
+   (see the media queries above). Letter labels are rendered
+   explicitly because an <ol>'s automatic numbers would disagree
+   with the in-text letter refs.
    ============================================================ */
 
-a.footnote-ref {
-    text-decoration: none;
-    color: var(--text-faint);
-    font-size: 0.75em;
-    line-height: 0;
+section.footnotes .footnotes-list {
+    list-style: none;
+    margin: 0;
+    padding: 0;
+}
+
+.footnote-item {
     position: relative;
-    top: -0.4em;
+    padding-left: 1.5rem;
+    margin-bottom: 0.85rem;
+    font-size: 0.85rem;
+    line-height: 1.6;
+    color: var(--text-muted);
+}
+
+.footnote-label {
+    position: absolute;
+    left: 0;
+    top: 0.15em;
     font-family: var(--font-sans);
+    font-size: 0.75em;
+    color: var(--text-faint);
+}
+
+/* First paragraph flows on the label's line; later ones stack. */
+.footnote-item > p {
+    margin: 0 0 0.5em;
+}
+.footnote-item > p:first-of-type {
+    display: inline;
+}
+
+.footnote-back {
+    margin-left: 0.35em;
+    text-decoration: none;
+    font-family: var(--font-sans);
+    color: var(--text-faint);
     transition: color var(--transition-fast);
 }
 
-a.footnote-ref:hover {
+.footnote-back:hover {
     color: var(--text-muted);
 }