[XeTeX] OT: scripts, glyphs and Cocoa text editing

[musa furber]

Both TeXShop and SubEthaEdit produce something like the image below:
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The render starts with Geeza Pro, all characters connected just as it  
should be. After the tilde it comes to Unicode 0xfda, a ligature  
missing in Geeza but present in other fonts. The renderer switches to  
a font that does have it (Lotus Linotype, from the looks of it) and  
then stays there. Lotus has OpenType tables for Arabic but not for  
AAT. My naive assumption is that this is why I get isolated shapes  
once the switch is made to Lotus.

It's nice that the rendering algorithm does what it can with what it  
is given. But it seems like it ought to take into account the font's  
script specific tables and not just its repertoire of glyphs.

Can anyone suggest a workaround, short of removing Lotus, editing  
Geeza, or making Al Bayan my default font? None of these solutions  
are ideal.

Regards,
Musa