I used to be able to do the following:

Press compose
Press '
Release '
Release compose
Press a

And the output would be á as I would expect.

At some point along the way, something changed and the character that comes instead is a ' and the next character is processed as if the compose key was pressed beforehand.

Press compose
Press ' ---> '
Release '
Release compose
Press a ---> nothing
Press a ---> å

The key to getting my á back is releasing the compose key before continuing the sequence:

Press compose
Release compose
Press '
Release '
Press a ----> á
Release a

I find that this makes typing these characters considerably harder, and I often have to make 4 attempts at typing them before getting what I want. It's even harder for characters like ñ.

After a few months of frustration I finally spotted how to restore the old behaviour: select the X Input Method (instead of Simple).

I'm not sure if I was previously using XIM and somehow switched to Simple (although I doubt it).

I also get my preferred behaviour on my Fedora 11 system (gtk+-2.16.6), but I'm not entirely sure which input method is being used there. The Input method choosing menu is not shown like it is on my Gentoo system, and $GTK_IM_MODULE is unset in the environment.

So, this is a bug report (or feature request) that the GTK simple IM module could be a little less fussy with compose sequences, and let us release the compose key after starting the rest of the sequence.