Version: (using Devel)
Installed from: Compiled sources
It looks like the tab bar will open new tabs on middle-click on MOUSE DOWN event, and close tabs on middle-click on MOUSE UP event, when middle-click is set up to do these.
So if you position the mouse on the tab bar to the right of the last opened tab, in the space where the new tab will appear and take up space, you're actually able to do a full middle click (down/up) where konqueror will create the new tab, then immediately close it, all from the single middle-click action. This is quick enough on my system to happen when clicking as fast as possible as long as the mouse is over the new tab's icon area, or click with a slight delay over the new title area. This behavior always occurs with a full tab bar cause middle-clickable tab area is so small.
KDE4 trunk r944788
Confirmed in KDE 4.3.1 and current trunk (if a URL is in the clipboard, such that middle clicking the empty area in the tab bar will open a new tab with that URL).
SVN commit 1039099 by kossebau:
changed: emit signal mouseMiddleClick() on release, not press of the mouse button
Matches behaviour of the signals "mouseMiddleClick( QWidget* )" and, from KTabBar, "mouseMiddleClick( int )", which the former is based on
M +9 -2 ktabwidget.cpp
M +1 -0 ktabwidget.h
WebSVN link: http://websvn.kde.org/?view=rev&revision=1039099
Also backported to 4.3, so fix should be in 4.3.3