I have compose set to spawn gvim. When gvim is open I can no longer focus the main claws-mail window. So I can not cut/paste things or access other emails until I finish the compose.
This can be solved by adding the -f option to the gvim commandline. Without -f, Claws Mail thinks gvim exits immediately and then blocks waiting for it. As of 8b99c246a19ac9281964d810c2f280b2ee799bfc the correct arguments are given as a one of the suggestions in the preferences (along with the xembed option).