content adapter: Fix issue with content starting out with a shortcode

Fixes #12544

parser/pageparser/pagelexer.go

@@ -62,7 +62,9 @@ func (l *pageLexer) Input() []byte {
 	return l.input
 }
 
-type Config struct{}
+type Config struct {
+	NoFrontMatter bool
+}
 
 // note: the input position here is normally 0 (start), but
 // can be set if position of first shortcode is known

parser/pageparser/pageparser.go

@@ -36,7 +36,11 @@ var _ Result = (*pageLexer)(nil)
 
 // ParseBytes parses the page in b according to the given Config.
 func ParseBytes(b []byte, cfg Config) (Items, error) {
-	l, err := parseBytes(b, cfg, lexIntroSection)
+	startLexer := lexIntroSection
+	if cfg.NoFrontMatter {
+		startLexer = lexMainSection
+	}
+	l, err := parseBytes(b, cfg, startLexer)
 	if err != nil {
 		return nil, err
 	}