diff --git a/internal/web/static/app.css b/internal/web/static/app.css
index 7b996e9..119ee8f 100644
--- a/internal/web/static/app.css
+++ b/internal/web/static/app.css
@@ -644,6 +644,10 @@ a.admin-link:hover { color: var(--fg); text-decoration: underline; }
   .stats-head { font-size: 13px; }
   .stats-grid { font-size: 13px; padding: 10px 12px; }
   .code { font-size: 13px; }
+  /* #167: gutter and code must share one line box at mobile width too — a
+     taller gutter font stacks its rows taller than the code rows and every
+     number drifts off its line start */
+  .code .gutter { font-size: 13px; }
   .codebody { padding: 0 12px; }
   .footnote { font-size: 12px; padding: 8px 12px; gap: 10px; }
   .iconbtn { font-size: 13px; padding: 6px 10px; }
diff --git a/internal/web/static/paste-lines.js b/internal/web/static/paste-lines.js
index 58b907b..231a473 100644
--- a/internal/web/static/paste-lines.js
+++ b/internal/web/static/paste-lines.js
@@ -1,7 +1,8 @@
 // #167: with line wrapping on, a logical line can occupy several visual
-// lines; the gutter must show one number per VISUAL line. Per-line spans
-// give each logical line its own box so its height identifies the wrapped
-// row count; blank gutter rows keep numbers aligned with line starts.
+// lines; the gutter must show one number per VISUAL line, and each number
+// must sit on the visual row where its logical line STARTS. Per-line spans
+// give each logical line its own box so offsetTop order stays correct even
+// when highlighting spans cross no line boundaries.
 (function () {
   var body = document.getElementById('codebody');
   var gutter = document.getElementById('gutter');
@@ -14,12 +15,9 @@
 
   // Wrap each logical line (split on newline; spans never contain newlines
   // because HighlightCode highlights per line) in a .codeline block. The
-  // blocks must be adjacent with NO newline text between them (#194):
-  // .codeline is display:block, and under pre-wrap each interleaved '\n'
-  // text node renders as an extra phantom row in the code column that
-  // renumber() does not count, drifting the gutter off alignment on
-  // wrapped lines. Blank lines become empty blocks; .codeline:empty keeps
-  // them one row tall (see app.css).
+  // spans are display:block, so they are joined with '' — a '\n' join leaves
+  // newline text nodes between blocks that pre-wrap renders as an extra line
+  // box per line, which would shift every following number down one row (#167).
   function splitLines() {
     var html = body.innerHTML;
     var parts = html.split('\n');
@@ -30,48 +28,85 @@
     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.
   function renumber() {
     var lines = body.querySelectorAll('.codeline');
-    var s = '';
-    if (wrapOn() && lines.length) {
-      // Each logical line block occupies rows = height / line-height when
-      // wrapped; its number sits on the first row and the remaining rows get
-      // blank gutter lines so numbers stay aligned with line starts.
-      // The gutter has a fixed CSS width now (.code .gutter), so its row
-      // count never changes the code column width: measurement is stable
-      // with no measurement/layout feedback.
-      var lh = parseFloat(getComputedStyle(body).lineHeight) || 1;
-      gutter.textContent = '';
-      var frag = document.createDocumentFragment();
-      for (var i = 0; i < lines.length; i++) {
-        var num = document.createElement('span');
-        num.className = 'gutline';
-        num.textContent = String(i + 1);
-        frag.appendChild(num);
-        var rows = Math.max(1, Math.round(lines[i].getBoundingClientRect().height / lh));
-        for (var r = 1; r < rows; r++) {
-          var blank = document.createElement('span');
-          blank.className = 'gutline';
-          blank.textContent = '\u00a0';
-          frag.appendChild(blank);
-        }
-      }
-      gutter.appendChild(frag);
-    } else {
+    if (!wrapOn() || !lines.length) {
+      // wrap OFF: one number per logical line (pre-existing behavior,
+      // including the gutter scrolling with horizontal scroll).
+      var s = '';
       for (var k = 1; k <= lines.length; k++) s += k + '\n';
       gutter.textContent = lines.length ? s.slice(0, -1) : '1';
+      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).
+    var bodyTop = body.getBoundingClientRect().top;
+    var lh = parseFloat(getComputedStyle(body).lineHeight) || 1;
+    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;
+    }
+    if (totalRows < lines.length) totalRows = lines.length;
+    gutter.textContent = '';
+    var frag = document.createDocumentFragment();
+    var spans = [];
+    for (var r = 0; r < totalRows; r++) {
+      var cell = document.createElement('span');
+      cell.className = 'gutline';
+      cell.textContent = '\u00a0';
+      spans.push(cell);
+      frag.appendChild(cell);
+    }
+    gutter.appendChild(frag);
+    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
+    // 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);
+    requestAnimationFrame(function () {
+      var moved = false;
+      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 (moved) renumber();
+    });
   }
 
   splitLines();
   renumber();
 
   // Re-renumber on toggle (theme.js toggles data-wrap on <html>) and on any
-  // size change (paste of lots of text, window resize, zoom).
+  // size change (paste of lots of text, window resize, zoom). Only width
+  // changes of the code column affect wrapping, so observe width only —
+  // height changes caused by our own renumbering must not re-trigger.
   var mo = new MutationObserver(renumber);
   mo.observe(document.documentElement, { attributes: true, attributeFilter: ['data-wrap'] });
+  var lastW = body.getBoundingClientRect().width;
   if (window.ResizeObserver) {
-    var ro = new ResizeObserver(function () { renumber(); });
+    var ro = new ResizeObserver(function () {
+      var w = body.getBoundingClientRect().width;
+      if (Math.abs(w - lastW) < 0.5) return;
+      lastW = w;
+      renumber();
+    });
     ro.observe(body);
   } else {
     window.addEventListener('resize', renumber);