Slashback Software

My (old) copy of MSDN shows:

WM_SIZING
  fwSide = wParam;         // edge of window being sized
  lprc = (LPRECT) lParam;  // screen coordinates of drag rectangle

which is identical to WM_MOVING.  Actually, the MSDN pages for WM_MOVING and WM_SIZING are nearly identical.  In fact, WM_MOVING uses the constants for WM_SIZING, e.g. WMSZ_BOTTOMLEFT.  I can't think why; I never seen a Windows window movable by anything but the titlebar.  Intrestingly, the Windows.pas unit has the WMSZ_BOTTOMLEFT et. al. constants, although I don't see them used elsewhere in the source.

TWMMoving from Messages.pas:

TWMMoving = packed record
  Msg: Cardinal;
  Edge: Integer;
  DragRect: PRect;
end;

which has no result.  I mean, MSDN says, "An application should return TRUE if it processes this message."  (That goes for either WM_MOVING or WM_SIZING.)

So here's TWMSizing:

type
  TWMSizing = packed record
    Msg: Cardinal;
    Edge: Integer;
    DragRect: PRect;
    Result: Longint;
  end;

This lets you do something like:

procedure WMSizing(var Message: TWMSizing); message WM_SIZING;
...
procedure TForm2.WMSizing(var Message: TWMSizing);
begin
  case Message.Edge of
    WMSZ_BOTTOMRIGHT: begin
      Message.DragRect.Bottom := Top + Height;  //can make it wider but not taller
      Message.Result := 1;  //return TRUE
    end;
    WMSZ_BOTTOMLEFT: begin
      Message.DragRect.Left := Left;  //can make it taller but not wider
      Message.Result := 1;  //return TRUE
    end;
  end;
end;