To install this, create a bookmark and copy and paste the above verbatim into there. To use it, first go to the conversation you want to single out in messenger.com, then navigate to the bookmarklet you created. The lefthand conversation list pane will vanish, and you'll be left with the IMs for your conversation in a window that has no minimum width (normally the conversation pane has a minimum width).
As fair warning, if Facebook updates the Messenger app then this hack may (probably will) stop working. This code is provided as-is and with no warranty whatsoever.