Subject: bug#20537: 25.0.50; `compose-string-internal` does not work properly on OSX.

From: Eli Zaretskii <[email-masked]>
Date: Sat, 09 May 2015 21:52:57 +0300

> Date: Sat, 09 May 2015 11:27:15 -0700
> From: Keith David Bershatsky <[email-masked]>
> 
> The function `compose-string-internal` does not work properly when running Emacs built on OSX.  The following example works on Windows, but not OSX -- it "should be" the letter A with a pipe running through the center of the letter A.  Depending upon the arguments passed to `compose-string-internal` when using Emacs on OSX, the pipe is either displayed to the right (at various distances) or is not visible at all -- it never displays "on top of" the target character.
> 
> (defun test-fn ()
> (interactive)
>   (message (compose-string-internal "A|" 0 (length "A|"))))

Character composition depends on the font driver doing TRT.  Which
font driver does your Emacs use, and does that driver support complex
text layout?