Stop passing invalid coordinates to ExtendOrCreateCurrentBlock()

This made the logic of this function unnecessarily more complicated.
Instead, just fall back to the current cell coordinates in the only
place where this could happen before.

Doing this still preserves the correct behaviour of Shift-arrow
selection when entire rows/columns are selected and the current cell is
not the leftmost/topmost cell (due to scrolling), but the code is
simpler.

Remove the now always true condition check and assert that it's indeed
always true.

Note that the changes to gridsel.cpp in this commit are best viewed
ignoring whitespace changes.

include/wx/generic/gridsel.h

@@ -73,9 +73,10 @@ public:
     // wxGrid::m_currentCellCoords (the exception is when we scrolled out from
     // the top of the grid and select a column or scrolled right and select
     // a row: in this case the lowest visible row/column will be set as
-    // current, not the first one). If the row or the column component of
-    // blockEnd parameter has value of -1, it means that the corresponding
-    // component of the current block should not be changed.
+    // current, not the first one).
+    //
+    // Both components of both blockStart and blockEnd must be valid.
+    //
     // Return true if the current block was actually changed or created.
     bool ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockStart,
                                     const wxGridCellCoords& blockEnd,

src/generic/grid.cpp

@@ -7925,12 +7925,17 @@ bool
 wxGrid::PrepareForSelectionExpansion(wxGridCellCoords& coords,
                                      const wxGridDirectionOperations& diroper)
 {
-    coords.SetRow(m_selection->GetCurrentBlockCornerRow());
-    coords.SetCol(m_selection->GetCurrentBlockCornerCol());
+    int row = m_selection->GetCurrentBlockCornerRow();
+    if ( row == -1 )
+        row = m_currentCellCoords.GetRow();
 
-    if ( coords == wxGridNoCellCoords )
-        coords = m_currentCellCoords;
-    else if ( !diroper.IsValid(coords) )
+    int col = m_selection->GetCurrentBlockCornerCol();
+    if ( col == -1 )
+        col = m_currentCellCoords.GetCol();
+
+    coords.Set(row, col);
+
+    if ( !diroper.IsValid(coords) )
     {
         // The component of the current block corner in our direction
         // is not valid. This means we can't change the selection block

src/generic/gridsel.cpp

@@ -486,6 +486,9 @@ bool wxGridSelection::ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockSt
                                                  const wxGridCellCoords& blockEnd,
                                                  const wxKeyboardState& kbd)
 {
+    wxASSERT( blockStart.GetRow() != -1 && blockStart.GetCol() != -1 &&
+              blockEnd.GetRow() != -1 && blockEnd.GetCol() != -1 );
+
     if ( m_selection.empty() )
     {
         SelectBlock(blockStart, blockEnd);
@@ -497,8 +500,6 @@ bool wxGridSelection::ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockSt
 
     bool editBlock = false;
 
-    if ( blockEnd.GetRow() != -1 )
-    {
     // If the new block starts at the same top row as the current one, the
     // end block coordinates must correspond to the new bottom row -- and
     // vice versa, if the new block starts at the bottom, its other end
@@ -513,26 +514,20 @@ bool wxGridSelection::ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockSt
         newBlock.SetTopRow(blockEnd.GetRow());
         editBlock = true;
     }
-    }
-    if ( blockEnd.GetCol() != -1 )
-    {
-        if ( newBlock.GetLeftCol() == blockStart.GetCol() )
+
+    if ( blockStart.GetCol() == block.GetLeftCol() )
     {
         newBlock.SetRightCol(blockEnd.GetCol());
         editBlock = true;
     }
-        else if ( newBlock.GetRightCol() == blockStart.GetCol() )
+    else if ( blockStart.GetCol() == block.GetRightCol() )
     {
         newBlock.SetLeftCol(blockEnd.GetCol());
         editBlock = true;
     }
-    }
 
     newBlock = newBlock.Canonicalize();
 
-    const bool endCoordsSet =
-        blockEnd.GetRow() != -1 && blockEnd.GetCol() != -1;
-
     if ( editBlock )
     {
         if ( newBlock == block )
@@ -562,15 +557,14 @@ bool wxGridSelection::ExtendOrCreateCurrentBlock(const wxGridCellCoords& blockSt
                                         true,
                                         kbd);
         m_grid->GetEventHandler()->ProcessEvent(gridEvt);
-        return true;
     }
-    else if ( endCoordsSet )
+    else
     {
         // Select the new one.
         SelectBlock(newBlock.GetTopLeft(), newBlock.GetBottomRight(), kbd);
-        return true;
     }
-    return false;
+
+    return true;
 }
 
 int wxGridSelection::GetCurrentBlockCornerRow() const