Hi,

[~]> zsh --version
zsh 5.0.2 (x86_64-suse-linux-gnu)


# When ls is not aliased, life is good
# Pressing tab switches between completions

[~]> alias ls
ls=_ls

[~]> ls <tab> bin/
bin/          config/       Desktop/      Documents/    Downloads/
 hacks/        public_html/  rpmbuild/

# Giving ls an alias pressing tab key shows all results on one line

[~]> alias ls="ls --color"
[~]> ls <tab> bin  config  Desktop  Documents  Downloads  hacks
 public_html  rpmbuild
<tab> bin  config  Desktop  Documents  Downloads  hacks  public_html
 rpmbuild