Fix auto generated header ids so they don't contain e.g. hyperlink destinations (note)

This makes the header ids match the newly added dt ids.

Also make sure newlines are preserved in hooks' `.PlainText`.

Fixes #13405
Fixes #13410

markup/goldmark/internal/extensions/attributes/attributes.go

@@ -1,6 +1,8 @@
 package attributes
 
 import (
+	"strings"
+
 	"github.com/gohugoio/hugo/markup/goldmark/goldmark_config"
 	"github.com/gohugoio/hugo/markup/goldmark/internal/render"
 	"github.com/yuin/goldmark"
@@ -181,12 +183,17 @@ func (a *transformer) generateAutoID(n ast.Node, reader text.Reader, pc parser.C
 }
 
 // Markdown settext headers can have multiple lines, use the last line for the ID.
-func textHeadingID(node *ast.Heading, reader text.Reader) []byte {
-	var line []byte
-	lastIndex := node.Lines().Len() - 1
-	if lastIndex > -1 {
-		lastLine := node.Lines().At(lastIndex)
-		line = lastLine.Value(reader.Source())
+func textHeadingID(n *ast.Heading, reader text.Reader) []byte {
+	text := render.TextPlain(n, reader.Source())
+	if n.Lines().Len() > 1 {
+
+		// For multiline headings, Goldmark's extension for headings returns the last line.
+		// We have a slightly different approach, but in most cases the end result should be the same.
+		// Instead of looking at the text segments in Lines (see #13405 for issues with that),
+		// we split the text above and use the last line.
+		parts := strings.Split(text, "\n")
+		text = parts[len(parts)-1]
 	}
-	return line
+
+	return []byte(text)
 }