I'm back to start again with GBL. Application start-up layout is as desired. However as soon as I draw upon "main page view panel" with paintComponent() page view panel does overlay thumbnail panel on the left.

Code for main panel to group up 3 main components

public class RottrMainPanel extends JPanel{

    private PdfManager pdfManager;

    public RottrMainPanel(){
        super();
        pdfManager = new PdfManager();
        setPanel();
        setBackground(Color.WHITE);
    }

    private void setPanel(){
        Dimension dim = new Dimensions().getMaxDimension();
        setLayout(new GridBagLayout());
        GridBagConstraints gbc = new GridBagConstraints();

        gbc.fill = GridBagConstraints.HORIZONTAL;
        gbc.anchor = GridBagConstraints.NORTHWEST;
        gbc.gridwidth = GridBagConstraints.REMAINDER;
        gbc.gridx = 0;
        gbc.gridy = 0;
        RottrToolBar rottrToolBar = new RottrToolBar(pdfManager);
        add(rottrToolBar, gbc);

        gbc.insets = new Insets(10, 10, 10, 10);
        gbc.fill = GridBagConstraints.VERTICAL;
        gbc.ipadx = 185;
        gbc.gridx = 0;
        gbc.gridy = 1;
        RottrThumbnailsPanel rottrThumbnailsPanel = new RottrThumbnailsPanel(pdfManager);
        JScrollPane jspThumbnails = new JScrollPane(rottrThumbnailsPanel,
                ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED,
                ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED);
        add(jspThumbnails, gbc);

        gbc.fill = GridBagConstraints.BOTH;
        gbc.gridx = 1;
        gbc.gridy = 1;
        gbc.weightx = 1.0;
        gbc.weighty = 1.0;
        RottrPageViewPanel rottrPageViewPanel = new RottrPageViewPanel(pdfManager);
        JScrollPane jspPageView = new JScrollPane(rottrPageViewPanel,
                ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED,
                ScrollPaneConstants.HORIZONTAL_SCROLLBAR_AS_NEEDED);
        add(jspPageView, gbc);


        dim = new Dimension((int)dim.getWidth()-10, (int)dim.getHeight()-10);
        setPreferredSize(dim);
        setMaximumSize(dim);
    }
}


and drawing on main view panel ( I do not think issue is here)

public void paintComponent(Graphics g) {
        //super.paintComponent(g);
        if (isDrawingAllowed) {
            g.drawImage(img, 0, 0, this);
        }
    }

    public void fileToOpen() {
        isDrawingAllowed = true;
        Pdf2Image pdf2Image = new Pdf2Image();
        img = pdf2Image.getPage(1, pdfManager.getPdfFile().toString());
        repaint();
    }

peter_budo,

As per code snippet, components are arranged in following layout

x:       0       1
y |-----------|------------|
0 |ToolBox | Thumb    |
   |---------- |------------|
1 |             |Rottr        |
   |------------------------|

Nope
gbc.gridx = 0;
gbc.gridy = 0;
RottrToolBar rottrToolBar = new RottrToolBar(pdfManager);

gbc.gridx = 0;
gbc.gridy = 1;
RottrThumbnailsPanel rottrThumbnailsPanel = new RottrThumbnailsPanel(pdfManager);

gbc.gridx = 1;
gbc.gridy = 1;
RottrPageViewPanel rottrPageViewPanel = new RottrPageViewPanel(pdfManager);

Reset gridwidth for second row.

Reset gridwidth for second row.

OK, and how I'm gone achieve width of that cell to be specific size?

You have to set width of a component which was placed into that cell. Do not use GridBagLayout to construct top level gui layout for your application. Use BorderLayout, GroupLayout, or BoxLayout.

A sample for review.

import java.awt.*;
import javax.swing.*;

public class P1 extends JFrame {
   public P1() {
      Tool a1=new Tool();
      Thumb a2=new Thumb(); 
      Rott a3=new Rott();
      GridBagLayout g=new GridBagLayout();
      GridBagConstraints gc=new GridBagConstraints();
      
      gc.gridwidth=gc.REMAINDER;
      gc.fill=gc.HORIZONTAL;
      gc.anchor=gc.NORTHWEST;
 
      getContentPane().setLayout(g);
      getContentPane().add(a1,gc);
      gc.gridheight=gc.REMAINDER; 
      gc.gridwidth=gc.RELATIVE;      

      gc.fill=gc.VERTICAL;
      gc.anchor=gc.NORTHWEST;

      gc.weighty=1.0;
      getContentPane().add(a2,gc);
      gc.gridwidth=gc.REMAINDER;
      gc.fill=gc.BOTH;
      gc.anchor=gc.NORTHWEST;
      getContentPane().add(a3,gc);
   
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      pack();
      setVisible(true);
   }
   public static void main(String []args) {
         new P1();
   }
}
class Tool extends JPanel{
   public Tool()    {
       setBackground(Color.RED);
       for(int i=0;i<=10;i++)
           add(new JButton("OK"));
     }
}
class Thumb extends JPanel{
   public Thumb()    {
       setBackground(Color.YELLOW);
for(int i=0;i<=2;i++)
           add(new JButton("OK"));
     }
}
class Rott extends JPanel{
   public Rott()    {
       setBackground(Color.BLUE);
for(int i=0;i<=3;i++)
           add(new JButton("OK"));
     }
}

I will have to play around with it as it is not giving exactly performance that I want...

It was original frame display mode that was playing havoc with my layout. Setting frame size based on Toolkit.getDefaultToolkit().getScreenSize(); doesn't put frame in max mod.
This was enforced by

GraphicsEnvironment e = GraphicsEnvironment.getLocalGraphicsEnvironment();
setMaximizedBounds(e.getMaximumWindowBounds());
setSize(e.getMaximumWindowBounds().width,e.getMaximumWindowBounds().height);
setExtendedState(getExtendedState() | JFrame.MAXIMIZED_BOTH);

Now frame layout is what I wanted from the start. Unfortunately resize have to be disabled as any change will mess up again. I'm not sure why this does happens....