Fix section linking and editing

canvas.go

@@ -298,8 +298,7 @@ func (h *handler) renderCanvasFile(v *canvasNodeView, n canvasNode) {
 		if heading := extractFirstHeading(raw); heading != "" {
 			v.Label = heading
 		}
-		v.Markdown = true
-		v.Body = template.HTML(convertMarkdown(canvasSubpath(raw, n.Subpath), dirBase(rel)))
+		canvasMarkdownBody(v, raw, n.Subpath, dirBase(rel))
 		return
 	}
 	u := fileURL(rel)
@@ -309,8 +308,7 @@ func (h *handler) renderCanvasFile(v *canvasNodeView, n canvasNode) {
 	switch {
 	case ext == ".md":
 		raw, _ := os.ReadFile(fsPath)
-		v.Markdown = true
-		v.Body = template.HTML(convertMarkdown(canvasSubpath(raw, n.Subpath), dirBase(path.Dir("/"+rel))))
+		canvasMarkdownBody(v, raw, n.Subpath, dirBase(path.Dir("/"+rel)))
 	case isImageFile(name) || ext == ".svg" || ext == ".webp":
 		thumb := u
 		if hasThumbnail(name) {
@@ -336,21 +334,36 @@ func dirBase(rel string) string {
 	return "/" + rel + "/"
 }
 
+// canvasMarkdownBody renders a markdown file node: the whole file, or the
+// section its subpath names. A subpath that names no heading says so on the
+// card rather than showing the whole file (as jsoncanvas-desktop does).
+func canvasMarkdownBody(v *canvasNodeView, raw []byte, subpath, base string) {
+	part, ok := canvasSubpath(raw, subpath)
+	if !ok {
+		v.Body = execNodeTemplate("canvas-missing", subpath)
+		return
+	}
+	v.Markdown = true
+	v.Body = template.HTML(convertMarkdown(part, base))
+}
+
 // canvasSubpath narrows markdown to a file node's "#Heading" subpath: that
-// heading's section including its subsections. Unknown subpaths (or block
-// references, "#^id") show the whole file.
-func canvasSubpath(raw []byte, subpath string) []byte {
+// heading's section including its subsections, matched case-insensitively
+// on the heading text. Without a subpath it is the whole file; ok is false
+// for a subpath that names no heading, which includes block references
+// ("#^id").
+func canvasSubpath(raw []byte, subpath string) (part []byte, ok bool) {
 	want := strings.TrimSpace(strings.TrimPrefix(subpath, "#"))
-	if want == "" || strings.HasPrefix(want, "^") {
-		return raw
+	if want == "" {
+		return raw, true
 	}
 	sections := splitSections(raw)
 	for i, s := range sections {
 		if _, text := sectionHeading(s); i > 0 && strings.EqualFold(text, want) {
-			return joinSections(sections[i:secionSpanEnd(sections, i)])
+			return joinSections(sections[i:secionSpanEnd(sections, i)]), true
 		}
 	}
-	return raw
+	return nil, false
 }
 
 // canvasTaskCheckboxes makes a text node's task checkboxes clickable. The