Ensure newlines between sections

sections.go

@@ -26,6 +26,15 @@ func splitSections(raw []byte) [][]byte {
 }
 
 // joinSections reassembles sections produced by splitSections.
+// Inserts a newline between sections when a non-empty section lacks a
+// trailing newline, so an edited section cannot inline the next heading.
 func joinSections(sections [][]byte) []byte {
-	return bytes.Join(sections, nil)
+	var buf bytes.Buffer
+	for i, s := range sections {
+		buf.Write(s)
+		if i < len(sections)-1 && len(s) > 0 && s[len(s)-1] != '\n' {
+			buf.WriteByte('\n')
+		}
+	}
+	return buf.Bytes()
 }