diff --git a/internal/web/static/paste-lines.js b/internal/web/static/paste-lines.js
index 231a473..41537f9 100644
--- a/internal/web/static/paste-lines.js
+++ b/internal/web/static/paste-lines.js
@@ -28,9 +28,11 @@
     body.innerHTML = out.join('');
   }
 
-  // One .gutline block per visual row. Numbers are placed at the gutter row
-  // whose top matches their .codeline's top; filler rows pad the gaps so
-  // alignment is driven by measured geometry, not by uniform row counts.
+  // One .gutline block per visual row. The gutter must reproduce the code
+  // column's REAL rendered rows: each number goes on the visual row whose top
+  // matches its .codeline's top (a line wrapping to N rows gets its number on
+  // the FIRST of those rows), and filler rows pad the gaps. Geometry is
+  // measured, not derived from span counts or heights.
   function renumber() {
     var lines = body.querySelectorAll('.codeline');
     if (!wrapOn() || !lines.length) {
@@ -42,11 +44,8 @@
       return;
     }
 
-    // Measure each logical line's geometry. Reading all rects first avoids
-    // interleaved layout reads/writes. A line's number goes on the visual row
-    // whose top matches the line's top; the gutter is sized to cover the
-    // deepest row any line STARTS on (a trailing empty logical line renders
-    // with zero height but still needs its number row).
+    // Read all rects first: batching layout reads before the writes below
+    // avoids interleaved read/write reflows.
     var bodyTop = body.getBoundingClientRect().top;
     var lh = parseFloat(getComputedStyle(body).lineHeight) || 1;
     var rowIdx = [];
@@ -76,15 +75,16 @@
     // settle a hair under N * line-height after the gutter is rebuilt; a
     // gutter pass that changes the code column width reflows it. Verify the
     // placement one frame later and re-run if any line's row moved (#167).
-    var placed = {};
-    for (var q = 0; q < tops.length; q++) placed[q] = Math.round((tops[q] - bodyTop) / lh);
+    var placed = [];
+    for (var p = 0; p < rowIdx.length; p++) placed.push(rowIdx[p]);
     requestAnimationFrame(function () {
       var moved = false;
+      var bodyTop2 = body.getBoundingClientRect().top;
+      var lh2 = parseFloat(getComputedStyle(body).lineHeight) || lh;
       var tops2 = [];
       for (var q2 = 0; q2 < lines.length; q2++) tops2.push(lines[q2].getBoundingClientRect().top);
-      var bodyTop2 = body.getBoundingClientRect().top;
       for (var q3 = 0; q3 < tops2.length; q3++) {
-        if (Math.round((tops2[q3] - bodyTop2) / lh) !== placed[q3]) { moved = true; break; }
+        if (Math.round((tops2[q3] - bodyTop2) / lh2) !== placed[q3]) { moved = true; break; }
       }
       if (moved) renumber();
     });