In developing a tool for processing text in exotic scripts, I'm having trouble choosing a GUI toolkit. The main part of the interface is to be a text editor, not much more elaborate than Notepad, but with its own input method editor. It is to be extensible in a scripting language so that non-programmers can develop their own input methods and display routines. It will be assumed that all files are UTF-8. More elaborate support like regexes is not needed.

The main sticking points are:

• characters beyond the Basic Multilingual Plane
• right-to-left and bi-directional text
• extension in a scripting language
• cross-platform Linux/Windows/OS X

My first choice was Tcl/Tk, but it lacks bidi and going beyond the BMP seems dodgy. At the other extreme, I've considered Qt with embedded ECMAScript, but that might be heavier and less malleable than I would like. I'm even thinking about making it browser based, but I'm concerned that the IM for large scripts would be too heavy for client-side processing. I've also looked at a few similar projects in Java, but the quality of the font rendering in SWING has been unacceptable.

What are your experiences in handling Unicode with various toolkits? Are there other serious issues I haven't considered? What would you recommend for doing this in the lightest way?