Drives the real Sidebar.Draw and reads the render counter, so a regression in
the draw path fails instead of a parallel calculation passing.
Three cases: one of two tabs active draws exactly one surface, a null active
tab draws zero, and icon-only mode still marks the active row.
Zero is a legitimate state, not a failure -- PickMainActiveTab returns null
when every tab is popped out, so the invariant is "at most one", not "exactly
one".
JonKazama-Hellion
5caa266287