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There are some "problems" with your code. First, do not use setSize (nor setPreferredSize) method of a component. Let the layout manager calculate its size and position.

Do not use Thread.sleep() in the Event Dispatch Thread. It will freeze the whole GUI. Since the thread sleeps, events cannot take place. Consider using a Swing Timer or a Swing Worker.

Because grid layout will give equal size to all of its components. It is actually kind of hard to get it, so I will try to explain it with an example.

Consider a panel with grid layout, with only 1 row and 10 columns. You try to add 10 components to it. This panel's width is equals to 100. Grid layout will give 10 width to each component, so 10x10 = 100.

Now, consider this panels width to be 102. How is gridlayout going to share it equally? It can't. So it will let 1 pixel empty from left, and 1 pixel from right. That's exactly you are facing. Since the GridLayout cannot share the width (and height) equally, it will let it empty. If you make the window bigger, at one moment the space will be enough to fit them equally:

See this .gif:

At the time when width is equals to 795-801, grid layout can not share the space equally to the 20 components a row has. When it goes 802, the margin that annoys you go away. That's because 20 components in row / 800 = 40 width for each component. +2 the left and right border (the green one).

The code that produces this behavior: