doc: add anchor links to headings

doc/Makefile

@@ -8,7 +8,7 @@
 # Pandoc flags
 FLAGS      = --filter pandoc-crossref --citeproc --toc --number-sections
 PDF_FLAGS  = --pdf-engine=xelatex
-HTML_FLAGS = --standalone --katex$(KATEX_URL)
+HTML_FLAGS = --standalone --katex$(KATEX_URL) --lua-filter filters/add-anchors.lua
 
 # Rebuild everything if the makefile changed
 .EXTRA_PREREQS += Makefile

doc/filters/add-anchors.lua

@@ -0,0 +1,14 @@
+-- Adds anchor links to headings with IDs
+function Header (h)
+  if h.identifier ~= '' then
+    -- an empty link to this header
+    local anchor_link = pandoc.Link(
+      {},                  -- content
+      '#' .. h.identifier, -- href
+      '',                  -- title
+      {class = 'anchor'}   -- attributes
+    )
+    h.content:insert(1, anchor_link)
+    return h
+  end
+end

doc/res/style.css

@@ -3,12 +3,35 @@
   box-sizing: border-box;
 }
 
+
 /* Make headings smaller */
 h1 { font-size: 1.8em }
 h2 { font-size: 1.4em }
 h3 { font-size: 1.1em }
 h1, h2, h3 { margin: 1em 0 }
 
+/* Heading anchors */
+.anchor::before {
+  content: '🔗';
+  font-size: 80%;
+  opacity: 0;
+  margin-right: -1.1em;
+  position: relative;
+  left: -1.2em;
+  z-index: 1;
+}
+.header-section-number,
+.anchor::before { transition: opacity 0.3s ease 0.1s }
+.anchor { text-decoration: none }
+
+h1:hover > .anchor::before,
+h2:hover > .anchor::before,
+h3:hover > .anchor::before { opacity: 1; }
+h1:hover > .header-section-number,
+h2:hover > .header-section-number,
+h3:hover > .header-section-number { opacity: 0; }
+
+
 /* Match math and normal mode fonts */
   body { font-size: 1.1em }
 .katex { font-size: 1.0em !important }