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    <p>You are probably implementing the message override for both of the grids. GridX scrolls GridY, which in turn scrolls GridX, which in turn ... SO. You can protect the superficial scrolling code by surrounding the block with flags.</p> <pre><code>type TForm1 = class(TForm) [..] private FNoScrollGridX, FNoScrollGridY: Boolean; [..] procedure TForm1.GridXCustomWndProc( var Msg: TMessage ); begin Msg.Result := CallWindowProc(POldWndProc, gridX.Handle, Msg.Msg, Msg.wParam, Msg.lParam ); if ( Msg.Msg = WM_VSCROLL ) then begin if not FNoScrollGridX then begin FNoScrollGridX := True gridY.SetActiveRow( gridX.GetActiveRow ); gridY.Perform( Msg.Msg, Msg.wParam, Msg.lParam ); // SetScrollPos( gridY.Handle, SB_VERT, HIWORD( Msg.wParam ), True ); end; FNoScrollGridX := False; end; end; </code></pre> <p>Similiar code for the GridY. BTW, you shouln't need the SetScrollPos. <hr> <b>edit:</b></p> <pre><code>TForm1 = class(TForm) [..] private GridXWndProc, GridXSaveWndProc: Pointer; GridYWndProc, GridYSaveWndProc: Pointer; procedure GridXCustomWndProc(var Msg: TMessage); procedure GridYCustomWndProc(var Msg: TMessage); [..] procedure TForm1.FormCreate(Sender: TObject); begin [..] GridXWndProc := classes.MakeObjectInstance(GridXCustomWndProc); GridXSaveWndProc := Pointer(GetWindowLong(GridX.Handle, GWL_WNDPROC)); SetWindowLong(GridX.Handle, GWL_WNDPROC, LongInt(GridXWndProc)); GridYWndProc := classes.MakeObjectInstance(GridYCustomWndProc); GridYSaveWndProc := Pointer(GetWindowLong(GridY.Handle, GWL_WNDPROC)); SetWindowLong(GridY.Handle, GWL_WNDPROC, LongInt(GridYWndProc)); end; procedure TForm1.GridXCustomWndProc(var Msg: TMessage); begin Msg.Result := CallWindowProc(GridXSaveWndProc, GridX.Handle, Msg.Msg, Msg.WParam, Msg.LParam); case Msg.Msg of WM_KEYDOWN: begin case TWMKey(Msg).CharCode of VK_UP, VK_DOWN, VK_PRIOR, VK_NEXT: GridY.Perform(Msg.Msg, Msg.WParam, Msg.LParam); end; end; WM_VSCROLL: GridY.Perform(Msg.Msg, Msg.WParam, Msg.LParam); WM_MOUSEWHEEL: begin ActiveControl := GridY; GridY.Perform(Msg.Msg, Msg.WParam, Msg.LParam); end; WM_DESTROY: begin SetWindowLong(GridX.Handle, GWL_WNDPROC, Longint(GridXSaveWndProc)); Classes.FreeObjectInstance(GridXWndProc); end; end; end; procedure TForm1.GridYCustomWndProc(var Msg: TMessage); begin Msg.Result := CallWindowProc(GridYSaveWndProc, GridY.Handle, Msg.Msg, Msg.WParam, Msg.LParam); case Msg.Msg of WM_KEYDOWN: begin case TWMKey(Msg).CharCode of VK_UP, VK_DOWN, VK_PRIOR, VK_NEXT: GridX.Perform(Msg.Msg, Msg.WParam, Msg.LParam); end; end; WM_VSCROLL: GridX.Perform(Msg.Msg, Msg.WParam, Msg.LParam); WM_MOUSEWHEEL: begin ActiveControl := GridX; GridY.Perform(Msg.Msg, Msg.WParam, Msg.LParam); end; WM_DESTROY: begin SetWindowLong(GridY.Handle, GWL_WNDPROC, Longint(GridYSaveWndProc)); Classes.FreeObjectInstance(GridYWndProc); end; end; end; </code></pre>
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    1. COI'm getting stack overflow errors when trying to use `inherited WndProc( Msg )` Also when I have the code as you have shown I get invalid reads of memory and other runtime errors. Not sure if this is the only way to accomplish the task?
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    2. COWhat `inherited WndProc`?, you gave a code part that replaces the `WindowProc` of a grid. Where do you override the 'WndProc'? Please edit the question to show the relevant code.
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    3. COMy apologies, see the edits
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