diff --git a/internal/web/static/paste-lines.js b/internal/web/static/paste-lines.js
index 41537f9..aacab75 100644
--- a/internal/web/static/paste-lines.js
+++ b/internal/web/static/paste-lines.js
@@ -20,6 +20,13 @@
   // box per line, which would shift every following number down one row (#167).
   function splitLines() {
     var html = body.innerHTML;
+    if (html.indexOf('\n') < 0) {
+      // No newline text nodes to split on (already joined in the server
+      // template or stripped upstream): blocks are already adjacent, so
+      // leave the DOM alone. Splitting here would clone the whole markup
+      // into one block and leave renumber() reading stale nodes (#205).
+      return;
+    }
     var parts = html.split('\n');
     var out = [];
     for (var i = 0; i < parts.length; i++) {