diff --git a/src/common/wincmn.cpp b/src/common/wincmn.cpp
index 7b458dca63..09a2f04668 100644
--- a/src/common/wincmn.cpp
+++ b/src/common/wincmn.cpp
@@ -446,8 +446,9 @@ void wxWindowBase::InvalidateBestSize()
     m_bestSizeCache = wxDefaultSize;
 
     // parent's best size calculation may depend on its children's
-    // best sizes, so let's invalidate it as well to be safe:
-    if (m_parent)
+    // best sizes, so let's invalidate it as well to be safe
+    // (but don't influence other wxTLWs)
+    if (m_parent && !IsTopLevel())
         m_parent->InvalidateBestSize();
 }