Yes this prevents the injection.

The injection is possible because the patch is substituted in the string without any escaping. Your fix is not enough. The real path to a Tix installation can contain special characters: '\', '{' or '}'.

My patch first sets a path to a Tcl variable (there is no an injection, because special API is used instead of evaluating a generated script), and then use this variable in the script (unlike to Unix shell Tcl doesn't reparse the command after substituting variables).