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auto-composition-mode and display properties

From: Stephen Berman
Subject: auto-composition-mode and display properties
Date: Thu, 21 Feb 2008 17:12:37 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/23.0.60 (gnu/linux)

Since the unicode merge, composite.el contains auto-composition-mode,
which is enabled globally by default.  This adds the auto-composed
property to every character.  The auto-composed property also gets added
to display properties.  For example:

1. emacs -Q
2. Type the string "ab".
3. Put the cursor after "b" and type 
   `M-: (put-text-property (1- (point)) (point) 'display "test")'.
Now the buffer display "atest".
4. Put the cursor after "a", i.e. on the first "t" and type `C-u C-x
='.  The *Help* buffer says:
     There are text properties here:
       auto-composed        t
       display              [Show]
By clicking on `[Show]' we get the value of the display property:
   #("test" 0 4
     (auto-composed t))

I assume the display property is supposed to be like this, i.e. this is
not a bug?  I ask because it is wreaking havoc with isearch in a mode I
have that uses display properties (e.g., isearch stops when it gets to
the bottom of the window, even though there are further occurrences out
of view, and the cursor cycles over lines above the last visible
occurrence).  There may be a bug in the mode that is triggered by
isearch with auto-composition-mode enabled, but I don't know how to
debug it: when I instrument isearch-mode for edebug and step through it,
after isearch-update is called the focus for entering isearch strings
becomes the buffer of isearch.el and switching back to the buffer with
my mode interrupts isearch.  So I cannot trigger the bug under edebug.
All I know is that when I disable auto-composition-mode, isearch works
fine in my mode (and it works fine in CVS Emacs prior to the unicode
merge).  So I would like to know, again, if auto-composition-mode is
really supposed to add the auto-composed property to the display string.
And does anyone have any suggestions for how I can debug isearch?

Steve Berman

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