POWERLEVEL9K_RPROMPT_ON_NEWLINE=false breaks completion menu

[romkatv]

This is on master.

To reproduce:

1. Enable completion menu.
2. Resize your terminal so that it has fewer than 20 lines (confirm with echo $LINES).
3. Set POWERLEVEL9K_PROMPT_ON_NEWLINE=true and POWERLEVEL9K_RPROMPT_ON_NEWLINE=false.
4. Type grep -- and press TAB twice.

Steps 1-3 more precisely (you still need to type grep -- and press TAB twice once ZSH prompt appears):

docker run -e LANG=C.UTF-8 -e LC_ALL=C.UTF-8 -e TERM=$TERM -it --rm ubuntu bash -uexc '
  cd
  apt update && apt install -y zsh git
  git clone https://github.com/bhilburn/powerlevel9k.git
  echo "
    zstyle :completion:\* menu yes select
    autoload -Uz compinit && compinit
    POWERLEVEL9K_PROMPT_ON_NEWLINE=true
    POWERLEVEL9K_RPROMPT_ON_NEWLINE=false
    source ~/powerlevel9k/powerlevel9k.zsh-theme" >~/.zshrc
  exec zsh'

Actual behavior:

Expected behavior:

[dritter]

Fixed on next with #1335