diff --git a/public/packets.js b/public/packets.js
index 12432e2c..5dcf3273 100644
--- a/public/packets.js
+++ b/public/packets.js
@@ -53,6 +53,7 @@
   let _displayPackets = [];       // filtered packets for current view
   let _displayGrouped = false;    // whether _displayPackets is in grouped mode
   let _rowCounts = [];            // per-entry DOM row counts (1 for flat, 1+children for expanded groups)
+  let _rowCountsDirty = false;    // set when _rowCounts may be stale (e.g. WS added children) (#410)
   let _cumulativeOffsetsCache = null; // cached cumulative offsets, invalidated on _rowCounts change
   let _lastVisibleStart = -1;     // last rendered start index (for dirty checking)
   let _lastVisibleEnd = -1;       // last rendered end index (for dirty checking)
@@ -396,6 +397,9 @@
                 existing._children.unshift(p);
                 if (existing._children.length > 200) existing._children.length = 200;
                 sortGroupChildren(existing);
+                // Invalidate row counts — child count changed, so virtual scroll
+                // heights are stale until next renderTableRows() (#410)
+                _invalidateRowCounts();
               }
             } else {
               // New group
@@ -442,6 +446,7 @@
     clearTimeout(_wsRenderTimer);
     _displayPackets = [];
     _rowCounts = [];
+    _rowCountsDirty = false;
     _cumulativeOffsetsCache = null;
     _observerFilterSet = null;
     _lastVisibleStart = -1;
@@ -1123,6 +1128,21 @@
       </tr>`;
   }
 
+  // Mark _rowCounts as stale so renderVisibleRows() recomputes them lazily.
+  // Called when expanded group children change outside renderTableRows() (#410).
+  function _invalidateRowCounts() {
+    _rowCountsDirty = true;
+    _cumulativeOffsetsCache = null;
+  }
+
+  // Recompute _rowCounts from _displayPackets if they've been invalidated.
+  function _refreshRowCountsIfDirty() {
+    if (!_rowCountsDirty || !_displayPackets.length) return;
+    _rowCounts = _displayPackets.map(function(p) { return _getRowCount(p); });
+    _cumulativeOffsetsCache = null;
+    _rowCountsDirty = false;
+  }
+
   // Compute the number of DOM <tr> rows a single entry produces.
   // Used by both row counting and renderVisibleRows to avoid divergence (#424).
   function _getRowCount(p) {
@@ -1161,6 +1181,9 @@
     const scrollContainer = document.getElementById('pktLeft');
     if (!scrollContainer) return;
 
+    // Recompute row counts if they were invalidated (e.g. WS added children) (#410)
+    _refreshRowCountsIfDirty();
+
     // Compute total DOM rows accounting for expanded groups
     const offsets = _cumulativeRowOffsets();
     const totalDomRows = offsets[offsets.length - 1];
@@ -1317,6 +1340,7 @@
     if (!displayPackets.length) {
       _displayPackets = [];
       _rowCounts = [];
+      _rowCountsDirty = false;
       _cumulativeOffsetsCache = null;
       _observerFilterSet = null;
       _lastVisibleStart = -1;
@@ -1336,6 +1360,7 @@
     _displayGrouped = groupByHash;
     _observerFilterSet = filters.observer ? new Set(filters.observer.split(',')) : null;
     _rowCounts = displayPackets.map(p => _getRowCount(p));
+    _rowCountsDirty = false;
     _cumulativeOffsetsCache = null;
 
     attachVScrollListener();
@@ -2044,6 +2069,8 @@
       renderPath,
       _getRowCount,
       _cumulativeRowOffsets,
+      _invalidateRowCounts,
+      _refreshRowCountsIfDirty,
       buildGroupRowHtml,
       buildFlatRowHtml,
     };
diff --git a/test-packets.js b/test-packets.js
index 94317b40..59586f17 100644
--- a/test-packets.js
+++ b/test-packets.js
@@ -757,6 +757,33 @@ console.log('\n=== packets.js: page registration ===');
   });
 }
 
+console.log('\n=== packets.js: _invalidateRowCounts / _refreshRowCountsIfDirty (#410) ===');
+{
+  const ctx = loadPacketsSandbox();
+  const api = ctx._packetsTestAPI;
+
+  test('_invalidateRowCounts and _refreshRowCountsIfDirty are exported', () => {
+    assert(typeof api._invalidateRowCounts === 'function');
+    assert(typeof api._refreshRowCountsIfDirty === 'function');
+  });
+
+  test('_invalidateRowCounts does not throw', () => {
+    api._invalidateRowCounts();
+  });
+
+  test('_refreshRowCountsIfDirty does not throw when no display packets', () => {
+    api._invalidateRowCounts();
+    api._refreshRowCountsIfDirty();
+  });
+
+  test('_cumulativeRowOffsets returns valid offsets after invalidation cycle', () => {
+    // Even with no display packets, should return valid array
+    const offsets = api._cumulativeRowOffsets();
+    assert(Array.isArray(offsets));
+    assert(offsets[0] === 0);
+  });
+}
+
 // ===== SUMMARY =====
 console.log(`\n${'='.repeat(40)}`);
 console.log(`packets.js tests: ${passed} passed, ${failed} failed`);