A reader of my article:

HOWTO: Detect when a Visual Studio toolwindow is shown or hidden.
http://www.mztools.com/articles/2011/MZ2011010.aspx

pointed me to an inconsistent behavior of the EnvDTE80.Events2.WindowVisibilityEvents in one case, so I have investigated all the cases and I have documented all the inconsistent behaviors (which are a lot in several VS versions) in a new article:

BUG: Inconsistent behavior of EnvDTE80.Events2.WindowVisibilityEvents from a Visual Studio add-in
http://www.mztools.com/articles/2012/MZ2012001.aspx

Since VS 11 Beta exhibits the same two bugs than VS 2010, I have opened two bug reports to Microsoft:

EnvDTE80.Events2.WindowVisibilityEvents.WindowHiding event not fired when collapsing toolwindow
https://connect.microsoft.com/VisualStudio/feedback/details/729827/envdte80-events2-windowvisibilityevents-windowhiding-event-not-fired-when-collapsing-toolwindow#details

EnvDTE80.Events2.WindowVisibilityEvents.WindowShowing event fired twice
https://connect.microsoft.com/VisualStudio/feedback/details/729845/envdte80-events2-windowvisibilityevents-windowshowing-event-fired-twice#details