Following the discussions in neovim/neovim#5649, I open this issue here as the upstream project. This is my original text from the neovim issue:

In case the completion is triggered close to the right border of the terminal, it might look something like this:

In some situations this makes the entries indistinguishable and completion pretty useless.

It would be nice if there was an option to specify a minimum width of the completion popup and in case the width cannot be held, the origin of the popup could be shifted to the left, away from the cursor position. As there is always only one of these popups, I don't see a strong reason why it always has to appear at the cursor location.

This has also been requested at the Debian bug tracker: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=450481 and @jamessan has indicated that an unfinished patch exists in the neovim issue.