TreeStyleTabBrowser#init() fails to set the attribute for the tabbar internal box,
so resizing of the element is unexpectedly tracked by BrowserUIShowHideObserver
even if it is triggered by the "auto hide" feature. It causes the bug #942.
When the mouse cursor moves onth the shrunken tab bar, autohide service expands
the width of the tab bar by setting width/height attribuets. Such a "resizing"
must be ignored by BrowserUIShowHideObserver because only unexpected resizing
caused by other addons triggers "update size of the floating tab bar".