Fix adding blocks to existing selection in wxGrid by Ctrl-drag.

Allow using Ctrl-dragging to add more blocks to the existing selection,
similarly to how spreadsheets work.

Closes #14141.

docs/changes.txt

@@ -585,6 +585,7 @@ Unix:
 All (GUI):
 
 - Fix hang when deleting columns from wxTreeListCtrl.
+- Allow using Ctrl-dragging to add to selection in wxGrid (Knut Petter Lehre).
 - Fix several floating point rounding bugs in wxPropertyGrid (Artur Wieczorek).
 - Restore support for wxFD_OVERWRITE_PROMPT and wxFD_FILE_MUST_EXIST in
   wxGenericFileDialog which was accidentally lost some time ago (Carl Godkin).

src/generic/grid.cpp

@@ -3953,7 +3953,9 @@ wxGrid::DoGridCellDrag(wxMouseEvent& event,
         case wxMOD_CONTROL:
             if ( m_selectedBlockCorner == wxGridNoCellCoords)
                 m_selectedBlockCorner = coords;
-            UpdateBlockBeingSelected(m_selectedBlockCorner, coords);
+            if ( isFirstDrag )
+                SetGridCursor(coords);
+            UpdateBlockBeingSelected(m_currentCellCoords, coords);
             break;
 
         case wxMOD_NONE: