React Aria's `CollectionBuilder` renders `TabList`'s children into both a
hidden collection-building tree and the real DOM. The hidden instance of
`TabsIndicators` sits outside the `TabListStateContext` provider, so its
`state` is `null` — causing its `updateCSSVariables` effect to hit the
`else` branch and write `--active-tab-opacity: 0` to the `tabsRef` DOM
element that the real instance also writes to. Under the right render
ordering, this hidden write lands after the real instance's `opacity: 1`
and makes the active indicator disappear on uncontrolled Tabs.
Guard `updateCSSVariables` with an early return when `state == null` so
the hidden instance never writes to the shared DOM element.
Signed-off-by: Johan Persson <johanopersson@gmail.com>
Johan Persson
c6fc76f532