What key symbol does Altgr-' give on your keyboard? ('xev' is a useful
program to check this) Why aren't you using simply <compose> a '? 

AltGr-' produces "{" on french keyboards.

« ' » produces keycode 13 (keysym 0x27, apostrophe), XLookupString gives 1
bytes:  "'"

« AltGr » produces keycode 113 (keysym 0xfe03, ISO_Level3_Shift), XLookupString
gives 0 bytes:  ""

« AltGr-' » produces keycode 13 (keysym 0x7b, braceleft), XLookupString gives 1
bytes:  "{"

I don't use <compose> a ' because this produces « á » not « à ».

If that can help:
« Alt-Gr-` » produces keycode 16 (keysym 0x60, grave), XLookupString gives 1
bytes:  "`"
« è » (the letter produced by a normal press on the key « 7 / è / `» ) produces
keycode 16 (keysym 0xe8, egrave), XLookupString gives 1 bytes:  "è"

Hmm. Probably a fr specific compose sequence. 

But doesn't the standard french keyboard *have* an à on the top row of the
keyboard?

The compose sequences that GTK+ supports are generic to all languages;
The ones that are supported for à are just the obvious:

 dead_grave + a
 compose + a + `
 compose + ` + a

Maybe while composing character, which was started with 'compose',
modifier key events like AltGr should be ignored?
 if (IsModiferKeys (keysym))
   ignore, and wait for next key event with ev->state = Mod1Mask

It may work if there is a compose sequence like below supported in 
default or Cedilla backend module.
  compose + a + AltGr<`>

I think what the reporter wants is to have

 compose + a + AltGr<'> act like compose + a + `

even though AltGr<'> gives { not `. Most likely the X compose
file for the fr locale just has a sequence for compose + a + {

Nono, I really want compose, a, AltGr<`> to act like compose, a, ` !!

There is no "`" key on french keyboards, but there's a "'" key. The problem is
that "`" is obtained only via AltGr, and this confuse the compose, a, ` sequence
in GTK. The sequence compose, e, ', works because AltGr isn't involved in typing
"'".

For you earlier remark, yes there's a "à" key on french keyboards, but no "À"
key, and many sentences start with this letter in french.

The OP is Roland Mas, and is reading this.

Regards,

-- 
Loic Minier

GTK+ doesn't pay any attention to modifiers during composition. 
If AltGr+` gives the 'grave' keysym, it really should work.

I can't test this out at the moment, since my keyboard doesn't have
enough keys to let me easily have both a compose key and a AltGR.

I think picking a french keyboard and setting caps lock as compose permits
testing (that's what I have, and it works on a US keyboard).

HTH,

-- 
Loic Minier

(In reply to comment #6)
> Nono, I really want compose, a, AltGr<`> to act like compose, a, ` !!
> 
> There is no "`" key on french keyboards, but there's a "'" key. The problem is
> that "`" is obtained only via AltGr, and this confuse the compose, a, ` sequence
> in GTK. The sequence compose, e, ', works because AltGr isn't involved in typing
> "'".
> 
> For you earlier remark, yes there's a "�" key on french keyboards, but no "�"
> key, and many sentences start with this letter in french.

It is quite strange to have  
        Compose, <letter>, diacritics

The typical is to have 
       Compose, diacritics, <letter>

Perhaps I am missing something.

On the issue. I managed to test on GTK+ (trunk) the following that work,

Compose, ', AltGr + ', a  produces ấ

which means that you can have a compose sequence that is made of a character/keysym that is produced with AltGr. This is how I understand the issue.

I think this is all fixed in trunk. Please reopen if not.