lfottaviano/emesene — comment on commit 6454d3ab0e97dd6884bc97bf3b53e0096a63aca3
File: emesene/gui/gtkui/ConversationManager.py line 73
By lfottaviano:

if the notebook receives a middle-click, this emits a button-press-event simulating that "left-button" was clicked.
If the notebook recvd a middle-click over a tab, there will be an implicit leftbutton event, Gtk.Notebook will intercept that signal changing the page.
And later will be removed.

It is true that if you middle-click any other place of Gtk.Notebook (this does not means everywhere) , will close the current tab, but this mades this function more interesting and comfortable too. This can be fixed with some condition. But remember, what i said, this is a "trick" and a rapid solution, if you dont like this way, i perfectly understand.