diff --git a/internal/web/static/paste-lines.js b/internal/web/static/paste-lines.js
index 02944a6..231a473 100644
--- a/internal/web/static/paste-lines.js
+++ b/internal/web/static/paste-lines.js
@@ -42,28 +42,22 @@
       return;
     }
 
-    // Measure each logical line's offsetTop (viewport-relative for
-    // comparison with gutter rows rendered in the same scroll flow).
-    // Reading all rects first avoids interleaved layout reads/writes.
-    var tops = [];
-    for (var i = 0; i < lines.length; i++) {
-      tops.push(lines[i].getBoundingClientRect().top);
-    }
-    // Build one gutline per visual row of the tallest column (the code
-    // body itself); each number is assigned to the visual row whose top
-    // is closest to its line's top.
+    // 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).
     var bodyTop = body.getBoundingClientRect().top;
     var lh = parseFloat(getComputedStyle(body).lineHeight) || 1;
-    var maxBottom = 0;
-    for (var j = 0; j < lines.length; j++) {
-      var btm = lines[j].getBoundingClientRect().bottom - bodyTop;
-      if (btm > maxBottom) maxBottom = btm;
+    var rowIdx = [];
+    var totalRows = 1;
+    for (var i = 0; i < lines.length; i++) {
+      var row = Math.round((lines[i].getBoundingClientRect().top - bodyTop) / lh);
+      if (row < 0) row = 0;
+      rowIdx.push(row);
+      if (row + 1 > totalRows) totalRows = row + 1;
     }
-    // Rows needed through the last visual row of the deepest line (ceil, so a
-    // partially-filled last row still gets its gutter span).
-    // Rows needed: floor(last line bottom / lh) + 1 — the row a line STARTS on
-    // must exist even when its bottom lands exactly on the body's edge.
-    var totalRows = Math.max(lines.length, Math.floor(maxBottom / lh) + 1);
+    if (totalRows < lines.length) totalRows = lines.length;
     gutter.textContent = '';
     var frag = document.createDocumentFragment();
     var spans = [];
@@ -75,11 +69,8 @@
       frag.appendChild(cell);
     }
     gutter.appendChild(frag);
-    for (var j = 0; j < tops.length; j++) {
-      var row = Math.round((tops[j] - bodyTop) / lh);
-      if (row < 0) row = 0;
-      if (row > totalRows - 1) row = totalRows - 1;
-      spans[row].textContent = String(j + 1);
+    for (var j = 0; j < rowIdx.length; j++) {
+      spans[rowIdx[j]].textContent = String(j + 1);
     }
     // A wrapped line whose last visual row is only partially filled can
     // settle a hair under N * line-height after the gutter is rebuilt; a