ZLE_RPROMPT_INDENT is not respected

[romkatv]

To reproduce:

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 "
    ZLE_RPROMPT_INDENT=1
    POWERLEVEL9K_PROMPT_ON_NEWLINE=true
    POWERLEVEL9K_MULTILINE_FIRST_PROMPT_PREFIX=
    POWERLEVEL9K_MULTILINE_LAST_PROMPT_PREFIX=
    source ~/powerlevel9k/powerlevel9k.zsh-theme" >~/.zshrc
  exec zsh'

Expected:

Note the empty space to the right of the prompt. This is due to ZLE_RPROMPT_INDENT=1.

Actual:

Note that the left and the right edges of the prompt are symmetric. This is in fact the expected behavior for ZLE_RPROMPT_INDENT=0 but it's not correct for ZLE_RPROMPT_INDENT=1.