My preferred method of entering characters outside of the ASCII range
(mostly European diacritics) is with the Compose key.  My Compose key is
the Pause key; this is configured by the following line in
/etc/default/keyboard on a Debian system:

XKBOPTIONS="terminate:ctrl_alt_bksp,compose:paus"

So, to enter "í" for example, I type (and release) Pause, then the "'"
key, and finally the "i" key.  This works everywhere - in old style X
programs like rxvt, in Gtk programs like Geeqie, in Emacs, in Firefox.

But not in Claws, at least not in the message search input box I get by
typing "/" when I'm browsing the message list.  Entering "í" as above
works, but not "á".  When I try to follow the procedure for entering
"á", I get a plain "a" instead, and to erase the "a" I need to hit
Backspace twice.

I suspect this has something to do with the extra keybindings that must
be in effect when the search box is focused, such as Escape for hiding
the search box.