UPDATE: From build 2198, there is a new setting "preview_on_click" for controlling the behaviour.
This have been commented many times in the past.
My plugin NoPreview stopped working and it is very difficult to use the SideBarEnhancements and SideBarGit packages when a preview tab popups.
A request with little improvements about the preview feature has been mentioned in viewtopic.php?p=16431#p16431
and viewtopic.php?p=23412#p23412
There is in-countable reports about this being confusing or useless in some circumstances.
viewtopic.php?f=4&t=5090
viewtopic.php?f=3&t=5812&p=25406&hilit=tab+bar#p25406
viewtopic.php?f=3&t=4590
http://stackoverflow.com/questions/9536 ... -a-new-tab
viewtopic.php?f=2&t=4768&start=0&hilit=preview
viewtopic.php?f=3&t=5175&start=0&hilit=preview
viewtopic.php?f=2&t=5065&start=0&hilit=preview
viewtopic.php?f=2&t=4834&start=0&hilit=preview
viewtopic.php?f=3&t=3196&p=15102&hilit=preview#p15102
viewtopic.php?f=2&t=4565&start=0&hilit=preview
viewtopic.php?f=3&t=4530&start=0&hilit=preview
viewtopic.php?f=3&t=3080&start=0&hilit=preview
viewtopic.php?f=3&t=2530&p=12357&hilit=preview#p12357
viewtopic.php?f=3&t=2487&p=11380&hilit=preview#p11380
viewtopic.php?f=3&t=3600&p=16969&hilit=preview#p16969
UPDATE: From build 2198, there is a new setting "preview_on_click" for controlling the behaviour.