Also avoid updating wxHeaderCtrl column when resizing in wxGrid

This is another attempt to get rid of the flicker when using the native
header control with wxGrid under MSW and avoid calling UpdateColumn(),
which is currently implemented in a very inefficient way in wxHeaderCtrl
under MSW, during interactive resizing.

See #18794.

include/wx/generic/private/grid.h

@@ -152,6 +152,14 @@ public:
                        (owner->CanHideColumns() ? wxHD_ALLOW_HIDE : 0) |
                        (owner->CanDragColMove() ? wxHD_ALLOW_REORDER : 0))
     {
+        m_inResizing = 0;
+    }
+
+    // Special method to call from wxGrid::DoSetColSize(), see comments there.
+    void UpdateIfNotResizing(unsigned int idx)
+    {
+        if ( !m_inResizing )
+            UpdateColumn(idx);
     }
 
 protected:
@@ -259,7 +267,20 @@ private:
 
     void OnResizing(wxHeaderCtrlEvent& event)
     {
+        // Calling wxGrid method results in a call to our own UpdateColumn()
+        // because it ends up in wxGrid::SetColSize() which must indeed update
+        // the column when it's called by the program -- but in the case where
+        // the size change comes from the column itself, it is useless and, in
+        // fact, harmful, as it results in extra flicker due to the inefficient
+        // implementation of UpdateColumn() in wxMSW wxHeaderCtrl, so skip
+        // calling it from our overridden version by setting this flag for the
+        // duration of this function execution and checking it in our
+        // UpdateIfNotResizing().
+        m_inResizing++;
+
         GetOwner()->DoHeaderDragResizeCol(event.GetWidth());
+
+        m_inResizing--;
     }
 
     void OnEndResize(wxHeaderCtrlEvent& event)
@@ -281,6 +302,9 @@ private:
 
     wxVector<wxGridHeaderColumn> m_columns;
 
+    // The count of OnResizing() call nesting, 0 if not inside it.
+    int m_inResizing;
+
     wxDECLARE_EVENT_TABLE();
     wxDECLARE_NO_COPY_CLASS(wxGridHeaderCtrl);
 };