source2e says “\twocolumn[BOX] : starts a new page, changing to twocolumn setting and puts BOX in a parbox of width \textwidth across the top.
Useful for full-width titles for double-column pages.
SURPRISE: The stretch from \@dbltextfloatsep will be inserted between the BOX and the top of the two columns. ”
But actually the stretch is NOT inserted. Why?
Page Layout ⇒ The stretch after \twocolumn[BOX]
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