Header Cut off in Pages 6.0
MacBook Air, macOS Sierra (10.12)
Whatever you do, do not hammer away at the tab key to position page numbers.
In Pages 5 & 6 it is sufficient to hit the tab key once for it to jump to the right of the Header/Footer and adopt a Right Align Tab for the page number. Or nothing if you don't type.
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Header Cut off in Pages 6.0