If there are more then 15 sites in piwik (and a user, for example the admin, is allowed to view them all), it is not possible to select all sites. the last sites are not selectable. because not all sites are being displayed in the selector.
this happens for example on the start page of piwik. on the vistor page all sites are selectable. because there is enough space.
(piwik is not customized, no new widgets or anything else)
this happens since version 0.2.26 Keywords: Website selector
Attachment: Patch to specify wmode=opaque (needed for older browsers or versions of Flash player). For FF3, you'll need to install Flash Player 10. 451.patch
Can you attach screenshots of where this works and doesn't work on your piwik installation?
Actually, I believe I see the problem now. You're referring to the bottom of the list not being selectable because these items are hidden behind a widget. This problem will also occur with the Language selector (dropdown list).
Yes. That is the problem.
fixed in  thanks for your patch! :)