Try following these steps exactly:
1) Open a new file in Paint.NET.
In the menu at the top -
Edit > Select All
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Edit > Erase Selection
You should now have one transparent (alpha) layer.
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2) This is important!
In the top-right corner of the Layers window,
click the red icon with the X inside it.
This closes the Layers window.
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3) In the Tools window, click the Line/Curve tool icon.
Set brush size to 20.
Draw a vertical line in the left part of the canvas,
similar to your previous screenshot.
Make a Rectangular Selection around the line.
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4) Menu:
Edit > Copy
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Edit > Paste in to New Layer
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5) The Move Selected Pixels tool is automatically in effect.
Move the selected pixels to the side with the mouse.