http://bugs.winehq.org/show_bug.cgi?id=15189
--- Comment #1 from Vincent Povirk madewokherd@gmail.com 2008-09-09 20:43:31 --- This seems to be because x11drv sends the window containing the one tab a WM_CANCELMODE message when it loses focus. Additionally, we lose the pointer grab when that window is unmapped, so even if the drag from the one window works, it's possible to move the mouse too fast to the upper-left so that the tab does not follow.