I managed to find a user manual for slickedit version 12 (2007) and it doesn't appear to have this option.
The following code might give you what you want. Create a file e.g. mymacros.e and load it using the load module command on the macro menu. The _switchbuf_my_show_spaces macro runs automatically whenever you switch buffers. The variable def_my_show_spaces controls whether there is no change (value 0), show spaces is enabled (1) or show spaces is disabled (2)). To set the value of def_my_show_spaces, use the set macro variable command on the macro menu (or the set-var command on the command line).
#include "slick.sh"
#pragma option(strictsemicolons,on)
#pragma option(strict,on)
#pragma option(autodecl,off)
#pragma option(strictparens,on)
int def_my_show_spaces;
void _switchbuf_my_show_spaces()
{
switch (def_my_show_spaces) {
default:
case 0 :
return;
case 1 :
// show spaces
if (p_ShowSpecialChars & SHOWSPECIALCHARS_SPACES)
return;
break;
case 2 :
// don't show spaces
if (!(p_ShowSpecialChars & SHOWSPECIALCHARS_SPACES))
return;
break;
}
p_ShowSpecialChars^=SHOWSPECIALCHARS_SPACES;
if (!_QReadOnly()) {
update_format_line();
}
}