Dear gang,

Within a font, relevant Unicode control characters such as the zero-width joiner usually have symbolic representation, but in printing the symbols are usually suppressed, while the effect of each character is maintained. 

In some text editors, such as Windows Notepad (Uniscribe), there is the option

"Show Unicode control characters"

I seem to recall that ConTeXt has/had a similar mechanism for toggling the symbols for Unicode control characters but cannot find it. 

Here is a MWE (note that the three control chars might be invisible in your email client):

==============

\startTEXpage[offset=1em]

\definedfont[almfixed at 14pt]

ZWJ: ‌

ZWNJ: ‍

NBSP:  

\stopTEXpage

==============

In Notepad, all three control characters can be turned on or off at once.

In Scintilla, only the NBSP appears and there is no option to toggle.

In ConTeXt, ZWJ and ZWJ appear as symbols, but NBSP does not.

Thank you in advance for your guidance.

Best wishes

Idris