tildify-space-string

tildify-space-string is a variable defined in `tildify.el'.
Its value is

" "

• This variable is safe as a file local variable if its value
  satisfies the predicate `t'.

Documentation:
Representation of a hard (a.k.a. no-break) space in current major mode.

Used by `tildify-buffer' in places where space is required but line
cannot be broken. For example "~" for TeX or " " for SGML,
HTML and XML modes. A no-break space Unicode character ("\u00A0")
might be used for other modes if compatible encoding is used.

If nil, current major mode has no way to represent a hard space.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 25.1 of Emacs.