Tabs selects the last tab on initial mount instead of the first

[Shinyaigeek]

Summary

<Tabs> from @openuidev/react-ui selects the last tab on initial mount instead of the first, even for a fully static (non-streaming) response.

Reproduction

Standalone repro (published packages only, npm install && npm run dev):
https://github.com/Shinyaigeek/openui-tabs-repro

root = Tabs([t1, t2, t3])
t1 = TabItem("a", "Apple", [c1])
t2 = TabItem("b", "Banana", [c2])
t3 = TabItem("c", "Cherry", [c3])
c1 = TextContent("Apple tab content")
c2 = TextContent("Banana tab content")
c3 = TextContent("Cherry tab content")

• Expected: the first tab (Apple) is active on mount.
• Actual: the last tab (Cherry) is active on mount.

Versions: @openuidev/react-ui@0.13.0, @openuidev/react-lang@0.2.8, @openuidev/react-headless@0.9.2, React 19.

Root cause

In packages/react-ui/src/genui-lib/Tabs/index.tsx, two effects race on mount:

1. An effect that defaults activeTab to the first tab.
2. A dependency-less effect that auto-advances to any tab whose serialized content grew since the last render (streaming follow-along):

for (const item of items) {
  const size = JSON.stringify(item.props.content).length;
  const prevSize = prevContentSizes.current[item.props.value] ?? 0; // {} on first run
  nextSizes[item.props.value] = size;
  if (size > prevSize) {
    candidate = item.props.value; // every tab "grew" -> candidate ends as the LAST tab
  }
}

On the very first pass prevContentSizes.current is empty, so every tab satisfies size > prevSize, and candidate ends the loop pointing at the last item. Both effects run in the same commit; the auto-advance effect runs after the default-to-first effect, so the last tab wins.

(SectionBlock guards its equivalent auto-advance with an isStreaming check; Tabs has no such guard.)

Suggested fix

Treat the first pass as a baseline-recording pass only: record nextSizes but skip candidate selection when prevContentSizes.current is empty. Streaming follow-along still works — subsequent growth (or a newly appearing tab) still advances the active tab.

I have a PR ready for this.