emacs -Q
M-x sql-postgres, enter connection details.
Type RET until prompt is at the window bottom.
Type something invalid as a command, like 'asdf', then RET.  Prompt changes to '->'.
Type semicolon, then RET.

The resulting output is in the attached screenshot.  The line with '6'
was the mode line, which got erased.

I've checked that it's not related to the patch in bug 33154 which I'm
currently testing.

Thanks.

In GNU Emacs 27.0.50 (build 3, x86_64-apple-darwin17.7.0, NS appkit-1561.60 Version 10.13.6 (Build 17G65))
 of 2018-11-02 built on fgunbin.playteam.ru
Repository revision: 5b218be0c362316384f5c9ef57a3bef02f742e94
System Description:  Mac OS X 10.13.6