Make menu dismissible
The ACP popup menu is sometimes too intrusive. I haven't found a reliable and convenient way to dismiss it.
<ESC> is not acceptable because it exits insert mode (and there is a delay before the menu effectively disappears.
Movement keys or typing text works, but are unacceptable if you don't want to type anything at this moment.
<Enter> is of course not acceptable if you don't want the completion.
Also, there seems to be a difference in behavior for the filename completion (no way to go back to original, so no way to enter a partial filename match at the end of a line)