I saw screen_top not at start of screen line, but at cursor location in middle of line.
VJ3ODCHRVOJ4AIAVOOVSCB64JB2DAIQ6UGV6JT2UTLWDCHNBLCRAC
UWNHC4AAO3SPOYLPANTO4WKCTZL7KAYC73Q2YUZFFW7K26FVJ7FQC
DLQMM2656JHXX3ONOEM6UIOXKFJFT5QT7RHWK7YS2W77PVZWHRSAC
OTIBCAUJ3KDQJLVDN3A536DLZGNRYMGJLORZVR3WLCGXGO6UGO6AC
assert(Screen_top1.line < Cursor1.line or (Screen_top1.line == Cursor1.line and Screen_top1.pos <= Cursor1.pos))