Ha, having just returned from the 'compose' frame to thread display after posting, it turns out that clicking the 'A' while composing did change the default font size, after all, even though it didn't return me to the compose window and displayed an error message.
Ha, having just returned from the 'compose' frame to thread display after posting, it turns out that clicking the 'A' while composing did change the default font size, after all, even though it didn't return me to the compose window and displayed an error message.