Why AndroWish switched to TCL_UTF_MAX=6

• Support of Emoji requires to cover the full range of Unicode code points (0x000000...0x10ffff).
• Unicode 8.0 support in Tcl was introduced by Jan Nijtmans during EuroTcl 2015.
• Emoji is quite popular on mobile devices, specifically fully supported on Android starting with the 4.4 release.
• There's a nice TrueType font named Symbola, which implements most of Emoji.
• By redefining TCL_UTF_MAX (default value 3) the valid range of supported code points can be adapted as well as the in-core memory requirements.
• Convention: TCL_UTF_MAX=3 and TCL_UTF_MAX=4 fits in 16 bit memory representation (sizeof (Tcl_UniChar)).
• Convention: TCL_UTF_MAX>4 needs 32 bit memory representation (sizeof (Tcl_UniChar)).
• TCL_UTF_MAX>=4 is able to cover the full range of code points (0x000000...0x10ffff).
• TCL_UTF_MAX=4 requires surrogate pairs internally to represent a code point larger than 0x00ffff by using two consecutive 16 bit values resembling UTF-8 notation.
• Usage of surrogate pairs is expensive and error prone with respect to counting the number of code points (and repeats a range of strange problems observed in the most popular programming languages of the year of this writing (whose names both start with the letter 'J')).
• TCL_UTF_MAX=6 forces usage of 32 bit as internal Tcl_UniChar representation and eliminates counting issues as well as escaping issues.
• 32 bit representation fits font rendering with freetype.

And last but not least:

• We are the Borg! AndroWish need not be compatible with nothing. You will be assimilated.

But there's hope:

Another article explaining the problem is The Tragedy of UCS-2 .