This package allows to surround a buffer with a one character border on all sides. This works on both graphic or terminal modes and takes advantage of margins, header-line, tab-line, mode-line, line-prefix and wrap-prefix.

If you display the same buffer in two different windows, the side borders will be either active or inactive in both windows (while the tab-line, header-line and mode-line should be ok). The reason is that line-prefix and wrap-prefix are attached to a buffer, not a window. I don't see any way to fix it.

If you have some text that has a display property spanning several lines, you'll see blanks on the sides. The reason is that the displayed property does not include the proper line-prefix / wrap-prefix from the main buffer. One way to fix it is to include a dynamic line-prefix / wrap-prefix inside your displayed text with the proper active/inactive glyph.

;; Immediate border toggling (using default header)
(require 'buffer-box)
(buffer-box)