diff --git a/internal/web/static/paste-lines.js b/internal/web/static/paste-lines.js
index 231a473..9853c3d 100644
--- a/internal/web/static/paste-lines.js
+++ b/internal/web/static/paste-lines.js
@@ -76,8 +76,8 @@
     // 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);
+    // rowIdx already holds each line's placement row for this pass.
+    var placed = rowIdx.slice();
     requestAnimationFrame(function () {
       var moved = false;
       var tops2 = [];