In order to lock the screen as you scroll:
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Go to the Window menu option
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Select "Split" from the options - this will let you place where you would like the diagram locked. Note that you can split the window into 2 identical panes, or into 4 panes.
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Go to the Window menu option
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Select "Freeze Panes"
You will notice that now as you start scrolling in your diagram, the image to the left, and/or bottom, does not move.