Here's an unfortunate new behavior which I didn't have time to report
or properly describe before.

When the next line starts on a character that has a distinct background,
that background is inherited when displaying newlines inside the display
string that renders the popup.

Here are two ways I usually trigger it:

1. Initiate completion on the "Author:" line of a LogEdit buffer (the
next line is an inverse-video line).

2. Have at least one space at the beginning of a line, and have
whitespace-mode on to highlight it in red. Then initiate completion on
the preceding line.

Screenshots are attached.

Needless to say, the latest Emacs 26 doesn't exhibit this problem.