I use SLY with GNU Emacs and SBCL on Linux Mint Cinnamon and I recently installed the sly-asdf and sly-quicklisp extensions.
Prior to installing the extensions, with the Emacs window maximized at full screen M-x sly opened the SLY buffer by splitting the Emacs window vertically. After installing the extensions, with the same Emacs window setup M-x sly splits the window horizontally.
Any insight into why? I haven't changed anything in the setup other than installing the extensions from MELPA.