The last reply says "twinline = yes" i.e. that there is a standing wave on
the open wire feeder, even though SWR is 1:1.

In the book Antenna Theory and Design by Robert S. Elliott, on page 60 there
is figure 2.2 titled "The dipole as a transmission line that is opened out".
The same figure is figure 1.15 on page 18 of Antenna Theory by Constatine
Balanis.

The figure shows that for a transmission line with the conductors close
together,
the fields cancel. The conductors are opened out into a dipole showing the
fields in phase. The diagram shows the current distribution on the dipole as
approximately sinusoidal. The high current area is at centre of dipole. The
diagram shows the current distribution sinewave on the dipole as also going
down the feeder.

Can someone comment on the reason the current sinewave is going down the
feeder? Perhaps in this example, the antenna is not resonant. I would have
expected a resonant antenna to have a flatline voltage and flatline current
on the feeder with only a standing wave on the dipole.

Is the current sinewave on the feeder a standing wave?

What is reason for standing wave not being on feeder? Do the forward and
reflected waves cancel at dipole centre? Are the waves in phase causing
resonance or a build up of signal, like pushing a swing in phase to get the
swing higher and higher?