The ToolWindow, and DocumentWindow for that matter, are only responsible for rendering their associated tab. So if there is no tab, then these elements are not considered visible. Their content is displayed by either a TabbedMdiContainer or a ToolWindowContainer. This is analogous to how the native WPF TabControl works (but in that case the tabs are always visible).
The only case where the ToolWindow/DocumentWindow is also responsible for display it's own content is when they are contained in a StandardMdiHost.