If this menu item is selected (that is, if it displays a check mark) in the Page Layout window, Finale won't let you drag a system so that it's superimposed on another system or beyond the page margins; nor will it let you drag the page margins beyond the boundaries of the page. It's a good idea to leave this option selected. Without it, you could accidentally drag a system so that it overlaps the preceding system - or is even off the page completely.
Page Layout Tool
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