diff --git a/internal/web/static/paste-lines.js b/internal/web/static/paste-lines.js
index a71d8ba..02944a6 100644
--- a/internal/web/static/paste-lines.js
+++ b/internal/web/static/paste-lines.js
@@ -54,7 +54,16 @@
     // is closest to its line's top.
     var bodyTop = body.getBoundingClientRect().top;
     var lh = parseFloat(getComputedStyle(body).lineHeight) || 1;
-    var totalRows = Math.max(lines.length, Math.ceil(body.getBoundingClientRect().height / lh));
+    var maxBottom = 0;
+    for (var j = 0; j < lines.length; j++) {
+      var btm = lines[j].getBoundingClientRect().bottom - bodyTop;
+      if (btm > maxBottom) maxBottom = btm;
+    }
+    // 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);
     gutter.textContent = '';
     var frag = document.createDocumentFragment();
     var spans = [];