Side panel tabs share a window, so which tab is shown is decided by view
z-order rather than the visibility flag (every tab in a window is
'visible'). Tests had no way to assert which tab is actually drawn in
front, which is distinct from which view has keyboard focus. Expose the
window's top view and add an IsActiveTab assertion built on it.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>