title: When centering dialog, first appears on topleft corner for some milliseconds
body: <p>I'm generating a dialog with a lot of content. I want to pack the dialog to the childrens height, and I want to center it on screen, so I'm doing this after adding all the childrens:</p>
  
  <pre><code>        shell.open();
          shell.layout();
          shell.pack(); //for wrap the dialog size to it's content width and height.
  
          //the dialog must be centered after doing shell.pack();
          Rectangle parentSize = getParent().getBounds();
          Rectangle shellSize = shell.getBounds();
          int x = parentSize.x + (parentSize.width - shellSize.width) / 2;
          int y = (int) (parentSize.y + (parentSize.height - shellSize.height) / 3.5);
          shell.setLocation(new Point(x, y));
  </code></pre>
  
  <p>The problem is that for some milliseconds the dialog is being visible with a lot of width and on the top left corner of the screen. I tried doing setVisible(false) and (true) to shell to do the trick but it doesn't works.</p>
  
  <p>How can I avoid to see the dialog on the top left corner and with a lot of width for some milliseconds?</p>
  
  <p>This is the full code:</p>
  
  <pre><code>public class SelectTwoMatrixNumbersDialog extends Dialog {
      protected ArrayList&lt;Integer&gt; matrixNumbers1;
      protected ArrayList&lt;Integer&gt; matrixNumbers2;
      protected Shell shell;
      private JointBet jointBet;
      private boolean result; 
      NumbersMatrixWidget numbersMatrixWidget1;
      NumbersMatrixWidget numbersMatrixWidget2;
  
      /**
       * Create the dialog.
       * @param parent
       * @param style
       */
      public SelectTwoMatrixNumbersDialog(Shell parent, int style, JointBet jointBet) {
          super(parent, style);
          setText("Elegir Números");
          this.jointBet = jointBet;
      }
  
      /**
       * Open the dialog.
       * @return the result
       */
      public boolean open() {
          createContents();
  
          matrixNumbers1 = new ArrayList&lt;Integer&gt;();
          matrixNumbers2 = new ArrayList&lt;Integer&gt;();
  
          GridLayout gl_shell = new GridLayout(1, false);
          gl_shell.marginWidth = 20;
          gl_shell.marginHeight = 20;
          shell.setLayout(gl_shell);
  
          Composite contentComposite = new Composite(shell, SWT.NONE);
          contentComposite.setLayout(new GridLayout(1, false));
          contentComposite.setLayoutData(new GridData(SWT.CENTER, SWT.CENTER, true, true, 1, 1));
  
          Label lblNumbers = new Label(contentComposite, SWT.NONE);
          lblNumbers.setLayoutData(new GridData(SWT.CENTER, SWT.CENTER, false, false, 1, 1));
          lblNumbers.setText("Números:");
  
          Composite numbersContainerComposite = new Composite(contentComposite, SWT.NONE);
          numbersContainerComposite.setLayoutData(new GridData(SWT.CENTER, SWT.CENTER, false, false, 1, 1));
  
          //variable values depending of type of game
          boolean firstCellEmpty = jointBet.getGameType().getNumbersMatrixArray()[0].isFirstCellEmpty();      
          int matrix1Rows = jointBet.getGameType().getNumbersMatrixArray()[0].getRows();
          int matrix1Columns = jointBet.getGameType().getNumbersMatrixArray()[0].getColumns();
          int matrix1MinNumber = jointBet.getGameType().getNumbersMatrixArray()[0].getMinNumber();
          int matrix1MaxNumber = jointBet.getGameType().getNumbersMatrixArray()[0].getMaxNumber();        
          GridLayout gl_numbersContainerComposite = new GridLayout(2, false);
          gl_numbersContainerComposite.horizontalSpacing = 15;
          numbersContainerComposite.setLayout(gl_numbersContainerComposite);
          numbersMatrixWidget1 = new NumbersMatrixWidget(numbersContainerComposite, SWT.BORDER, false, jointBet.getGameType().getNumbersMatrixArray()[0].getNumbers(), matrix1Rows, matrix1Columns, matrix1MinNumber, matrix1MaxNumber, firstCellEmpty);
  
          //variable values depending of type of game
          firstCellEmpty = jointBet.getGameType().getNumbersMatrixArray()[1].isFirstCellEmpty();      
          int matrix2Rows = jointBet.getGameType().getNumbersMatrixArray()[1].getRows();
          int matrix2Columns = jointBet.getGameType().getNumbersMatrixArray()[1].getColumns();
          int matrix2MinNumber = jointBet.getGameType().getNumbersMatrixArray()[1].getMinNumber();
          int matrix2MaxNumber = jointBet.getGameType().getNumbersMatrixArray()[1].getMaxNumber();
          numbersMatrixWidget2 = new NumbersMatrixWidget(numbersContainerComposite, SWT.BORDER, false, jointBet.getGameType().getNumbersMatrixArray()[1].getNumbers(), matrix2Rows, matrix2Columns, matrix2MinNumber, matrix2MaxNumber, firstCellEmpty);
          numbersMatrixWidget2.setLayoutData(new GridData(SWT.LEFT, SWT.BOTTOM, false, false, 1, 1));
  
          Composite fillNumbersComposite = new Composite(shell, SWT.NONE);
          fillNumbersComposite.setLayoutData(new GridData(SWT.CENTER, SWT.CENTER, false, false, 1, 1));
          fillNumbersComposite.setLayout(new GridLayout(1, false));
  
          Composite compositeFillMatrix1 = new Composite(fillNumbersComposite, SWT.NONE);
          compositeFillMatrix1.setLayout(new GridLayout(4, false));
  
          Label lblFillMatrix1WithMostCommonNumbers1 = new Label(compositeFillMatrix1, SWT.NONE);
          lblFillMatrix1WithMostCommonNumbers1.setText("Rellenar Matriz 1 con los");
  
          Spinner matrix1NumbersSpinner = new Spinner(compositeFillMatrix1, SWT.BORDER);
          matrix1NumbersSpinner.setMaximum(jointBet.getGameType().getNumbersMatrixArray()[0].getPossibleNumbersCount());
          matrix1NumbersSpinner.setMinimum(1);
  
          Label lblFillMatrix1WithMostCommonNumbers2 = new Label(compositeFillMatrix1, SWT.NONE);
          lblFillMatrix1WithMostCommonNumbers2.setText("números mas repetidos:");
  
          Button btnFillMatrix1 = new Button(compositeFillMatrix1, SWT.NONE);
          btnFillMatrix1.setText("Rellenar");
          btnFillMatrix1.addSelectionListener(new SelectionAdapter() {
              @Override
              public void widgetSelected(SelectionEvent e) {
                  String gameType = jointBet.getGameType().getType();
                  int matrix = 1;
                  int amount = Integer.parseInt(matrix1NumbersSpinner.getText());
                  numbersMatrixWidget1.setNumbers(StatisticsManager.getInstance().getMostUsedNumberKeys(gameType, matrix, amount));
              }
          });
  
          Composite compositeFillMatrix2 = new Composite(fillNumbersComposite, SWT.NONE);
          compositeFillMatrix2.setLayout(new GridLayout(4, false));
  
          Label lblFillMatrix2WithMostCommonNumbers1 = new Label(compositeFillMatrix2, SWT.NONE);
          lblFillMatrix2WithMostCommonNumbers1.setText("Rellenar Matriz 2 con los");
  
          Spinner matrix2NumbersSpinner = new Spinner(compositeFillMatrix2, SWT.BORDER);
          matrix2NumbersSpinner.setMaximum(jointBet.getGameType().getNumbersMatrixArray()[1].getPossibleNumbersCount());
          matrix2NumbersSpinner.setMinimum(1);
  
          Label lblFillMatrix2WithMostCommonNumbers2 = new Label(compositeFillMatrix2, SWT.NONE);
          lblFillMatrix2WithMostCommonNumbers2.setText("números mas repetidos:");
  
          Button btnFillMatrix2 = new Button(compositeFillMatrix2, SWT.NONE);
          btnFillMatrix2.setText("Rellenar");
          btnFillMatrix2.addSelectionListener(new SelectionAdapter() {
              @Override
              public void widgetSelected(SelectionEvent e) {
                  String gameType = jointBet.getGameType().getType();
                  int matrix = 2;
                  int amount = Integer.parseInt(matrix2NumbersSpinner.getText());
                  numbersMatrixWidget2.setNumbers(StatisticsManager.getInstance().getMostUsedNumberKeys(gameType, matrix, amount));
              }
          });
  
          Composite buttonsComposite = new Composite(shell, SWT.NONE);
          FillLayout fl_buttonsComposite = new FillLayout(SWT.HORIZONTAL);
          fl_buttonsComposite.marginHeight = 10;
          fl_buttonsComposite.spacing = 40;
          buttonsComposite.setLayout(fl_buttonsComposite);
          GridData gd_buttonsComposite = new GridData(SWT.CENTER, SWT.CENTER, false, false, 1, 1);
          gd_buttonsComposite.heightHint = 45;
          buttonsComposite.setLayoutData(gd_buttonsComposite);
  
          Button acceptButton = new Button(buttonsComposite, SWT.NONE);
          acceptButton.addSelectionListener(new SelectionAdapter() {
              @Override
              public void widgetSelected(SelectionEvent e) {  
                  //matrix 1
                  matrixNumbers1.clear();
                  for (Button b : numbersMatrixWidget1.getButtons()) {
                      if (b.getSelection()) {
                          matrixNumbers1.add(Integer.parseInt(b.getText()));
                      }
                  }
  
                  //matrix 2
                  matrixNumbers2.clear();
                  for (Button b : numbersMatrixWidget2.getButtons()) {
                      if (b.getSelection()) {
                          matrixNumbers2.add(Integer.parseInt(b.getText()));
                      }
                  }
  
                  if (matrixNumbers1.size()&lt;jointBet.getGameType().getNumbersMatrixArray()[0].getNumbersPerBet() || matrixNumbers2.size()&lt;jointBet.getGameType().getNumbersMatrixArray()[1].getNumbersPerBet()) {
                      ErrorDialog dialog = new ErrorDialog(shell, SWT.DIALOG_TRIM | SWT.PRIMARY_MODAL, "El mínimo de números es "+jointBet.getGameType().getNumbersMatrixArray()[0].getNumbersPerBet()+" + "+jointBet.getGameType().getNumbersMatrixArray()[1].getNumbersPerBet());
                      dialog.open();
                  }else {                 
                      Collections.sort(matrixNumbers1);
                      Collections.sort(matrixNumbers2);
  
                      jointBet.setNumbers(0, matrixNumbers1);
                      jointBet.setNumbers(1, matrixNumbers2);
                      DataManager.getInstance().saveData();
  
                      result = true;
                      shell.close();
                  }
              }
          });
          acceptButton.setBounds(0, 0, 75, 25);
          acceptButton.setText("Aceptar");
  
          Button cancelButton = new Button(buttonsComposite, SWT.NONE);
          cancelButton.addSelectionListener(new SelectionAdapter() {
              @Override
              public void widgetSelected(SelectionEvent e) {
                  result = false;
                  shell.close();
              }
          });
          cancelButton.setBounds(0, 0, 75, 25);
          cancelButton.setText("Cancelar");
  
          shell.setDefaultButton(acceptButton);
  
          shell.open();
          shell.layout();
          shell.pack(); //for wrap the dialog size to it's content width and height.
  
          //the dialog must be centered after doing shell.pack();
          Rectangle parentSize = getParent().getBounds();
          Rectangle shellSize = shell.getBounds();
          int x = parentSize.x + (parentSize.width - shellSize.width) / 2;
          int y = (int) (parentSize.y + (parentSize.height - shellSize.height) / 3.5);
          shell.setLocation(new Point(x, y));
  
          Display display = getParent().getDisplay();
          while (!shell.isDisposed()) {
              if (!display.readAndDispatch()) {
                  display.sleep();
              }
          }
  
          return result;
      }
  
      /**
       * Create contents of the dialog.
       */
      private void createContents() {
          shell = new Shell(getParent(), getStyle());
          shell.setText(getText());
      }
  }
  </code></pre>