Perhaps I've mistaken the bug. Starting SLIME now opens a scratch window and buffer, and I don't think it did before. It used to have a full-size window showing all REPL output. But now it splits the window into a scratch window and the REPL. That means the compiler output goes into the scratch window, which will not close with a Q keystroke. If I close the scratch window after slime starts but before loading the system, the problem does not occur.
it sounds strange. on linux, XEmacs 21.4 (patch 19), sbcl cvs, some fresh slime i get a full window of the repl buffer.