Oops, it isn't YahooPops that lets you choose which webmail UI to match
on at the Yahoo Mail web site. Must've been some other screen-scraper
utility, like PopPeeper. In the 0.8 version (that I just looked at in a
virtual machine), there is no option to have it match its navigational
screen set to the one you select for Yahoo Mail's webmail interface.
Ypops must've been encoded to try to guess which webmail UI on which to
match. So switch Yahoo Mail's webmail UI back to the classic view and
retest.