Fix preserving selection when changing selection mode in wxGrid

The loop over the existing selection was buggy and took into account only one
corner of the block instead of the entire block and also skipped some blocks
entirely.

Closes #17572.

docs/changes.txt

@@ -90,6 +90,7 @@ All (GUI):
 - Add wxListCtrl::SetHeaderAttr().
 - Add support for using markup in wxDataViewCtrl text items.
 - Implement auto complete in generic wxSearchCtrl (Eric Jensen).
+- Fix preserving selection when changing selection mode in wxGrid (jonkraber).
 
 wxGTK:
 

src/generic/gridsel.cpp

@@ -144,14 +144,15 @@ void wxGridSelection::SetSelectionMode( wxGrid::wxGridSelectionModes selmode )
         }
 
         // Note that m_blockSelectionTopLeft's size may be changing!
-        for (n = 0; n < m_blockSelectionTopLeft.GetCount(); n++)
+        for ( n = m_blockSelectionTopLeft.GetCount(); n > 0; )
         {
+            n--;
             wxGridCellCoords& coords = m_blockSelectionTopLeft[n];
             int topRow = coords.GetRow();
             int leftCol = coords.GetCol();
-            coords = m_blockSelectionBottomRight[n];
-            int bottomRow = coords.GetRow();
-            int rightCol = coords.GetCol();
+            wxGridCellCoords& coords2 = m_blockSelectionBottomRight[n];
+            int bottomRow = coords2.GetRow();
+            int rightCol = coords2.GetCol();
 
             if (selmode == wxGrid::wxGridSelectRows)
             {