Outer borders are never shown on text terminal frames and on frames generated by GTK+ routines. On MS-Windows, the outer border is emulated with the help of a one pixel wide external border. Non-toolkit builds on X allow to change the color of the outer border by setting the border-color frame parameter.
我在 Linux 上折腾过一些 posframe 之类的插件也有这个问题,后来就不用了(个人经常用终端版 emacs)。想要通用的话其实可以改一下 internal border 的颜色,这样边框会是紧贴着文字外边的。