Created attachment 7303 Panel with partial frame The panel on 4.13.1 shows a frame around window buttons and Application menu, but not around Whisker Menu, Launcher and other items, just to give an example. Take a look at the screenshots showing the panel and the configured panel items.
Created attachment 7304 Configured panel items
Usiong Gtk2 theme /usr/share/themes/MurrinaCandido/gtk-2.0/gtkrc from http://sunsite.informatik.rwth-aachen.de/ftp/pub/linux/gentoo/distfiles/MurrinaCandido.tar.gz
Strange, I can't reproduce that with 4.13.1 with Adwaita. The border goes all the way through and works for whiskermenu just as well as the internal application menu. Please note that 4.13.1 is Gtk+3, so your Gtk+2 theme won't have any effect there! Could you try with Adwaita? (That is the Gtk+3 standard/reference theme)
Switching to Adwaita doesn't have any effect on the panel look, and the issue is still the same, also after xfce4-panel --restart.
Could you please test this again with the latest panel release? I still haven't found any way to reproduce this. Alternatively you could also try to provide your panel configuration with xfpanel-switch/xfce4-panel-profiles and attach the tarball as an attachment to this report.
I can't reproduce anymore. Eventhough not using Adwaita today, switching back to Adwaita for a preview does not show the effect anymore.
Great, thanks for checking again!