“I think it would be very confusing for the user, that “page down” is only half a screen sometimes …”
Well,
I`m not sure what is so confusing about such a screen (please see below)? It simply means “NO MORE DATA”, right?
Any way, I was never confused while seeing the line
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in the middle (or nearly at the beginning of the screen view) 
//CMWKF02 DD DSN=&&INTA1,DISP=(OLD, DELETE)
//**
//CMWKF03 DD DSN=TSTSPRH.SALIND.MVMNT1,
// DISP=(NEW,CATLG,DELETE),SPACE=(TRK,(6,1),RLSE)
//**
//SYSUT2 DD SYSOUT=*
//CMSYNIN DD *
LOGON POSTES
VSOPEN ALL
UPDATE OFF
RP0280U
FIN
/*
//
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