diff --git a/include/wx/msw/listbox.h b/include/wx/msw/listbox.h
index 4a78e165a2..54a53b1c58 100644
--- a/include/wx/msw/listbox.h
+++ b/include/wx/msw/listbox.h
@@ -145,6 +145,8 @@ public:
     // returns true if the platform should explicitly apply a theme border
     virtual bool CanApplyThemeBorder() const { return false; }
 
+    virtual void OnInternalIdle();
+
 protected:
     virtual wxSize DoGetBestClientSize() const;
 
@@ -164,8 +166,8 @@ protected:
     // this can't be called DoHitTest() because wxWindow already has this method
     virtual int DoHitTestList(const wxPoint& point) const;
 
-    bool m_updateHorizontalExtent;
-    virtual void OnInternalIdle();
+    // free memory (common part of Clear() and dtor)
+    void Free();
 
     unsigned int m_noItems;
 
@@ -175,6 +177,14 @@ protected:
 #endif
 
 private:
+    // call this when items are added to or deleted from the listbox or an
+    // items text changes
+    void MSWOnItemsChanged();
+
+    // flag indicating whether the max horizontal extent should be updated,
+    // i.e. if we need to call SetHorizontalExtent() from OnInternalIdle()
+    bool m_updateHorizontalExtent;
+
     DECLARE_DYNAMIC_CLASS_NO_COPY(wxListBox)
 };
 
diff --git a/src/msw/listbox.cpp b/src/msw/listbox.cpp
index 78d76fdfab..d36f5ab109 100644
--- a/src/msw/listbox.cpp
+++ b/src/msw/listbox.cpp
@@ -186,7 +186,6 @@ bool wxListBox::Create(wxWindow *parent,
     }
 
     // now we can compute our best size correctly, so do it again
-    InvalidateBestSize();
     SetInitialSize(size);
 
     return true;
@@ -269,6 +268,22 @@ void wxListBox::OnInternalIdle()
     }
 }
 
+void wxListBox::MSWOnItemsChanged()
+{
+    // we need to do two things when items change: update their max horizontal
+    // extent so that horizontal scrollbar could be shown or hidden as
+    // appropriate and also invlaidate the best size
+    //
+    // updating the max extent is slow (it's an O(N) operation) and so we defer
+    // it until the idle time but the best size should be invalidated
+    // immediately doing it in idle time is too late -- layout using incorrect
+    // old best size will have been already done by then
+
+    m_updateHorizontalExtent = true;
+
+    InvalidateBestSize();
+}
+
 // ----------------------------------------------------------------------------
 // implementation of wxListBoxBase methods
 // ----------------------------------------------------------------------------
@@ -297,8 +312,7 @@ void wxListBox::DoDeleteOneItem(unsigned int n)
     SendMessage(GetHwnd(), LB_DELETESTRING, n, 0);
     m_noItems--;
 
-    // SetHorizontalExtent(wxEmptyString); can be slow
-    m_updateHorizontalExtent = true;
+    MSWOnItemsChanged();
 
     UpdateOldSelections();
 }
@@ -328,7 +342,7 @@ void wxListBox::DoClear()
     ListBox_ResetContent(GetHwnd());
 
     m_noItems = 0;
-    m_updateHorizontalExtent = true;
+    MSWOnItemsChanged();
 
     UpdateOldSelections();
 }
@@ -486,7 +500,7 @@ int wxListBox::DoInsertItems(const wxArrayStringsAdapter & items,
         AssignNewItemClientData(n, clientData, i, type);
     }
 
-    m_updateHorizontalExtent = true;
+    MSWOnItemsChanged();
 
     UpdateOldSelections();
 
@@ -537,7 +551,7 @@ void wxListBox::SetString(unsigned int n, const wxString& s)
     if ( wasSelected )
         Select(n);
 
-    m_updateHorizontalExtent = true;
+    MSWOnItemsChanged();
 }
 
 unsigned int wxListBox::GetCount() const
@@ -551,9 +565,6 @@ unsigned int wxListBox::GetCount() const
 
 void wxListBox::SetHorizontalExtent(const wxString& s)
 {
-    // in any case, our best size could have changed
-    InvalidateBestSize();
-
     // the rest is only necessary if we want a horizontal scrollbar
     if ( !HasFlag(wxHSCROLL) )
         return;