Scroll-view: honour auto_scroll_to, gap on Android, and chrome collapse

[shanerbaner82]

Three scroll-view renderer gaps, all surfaced while building a chat screen.

Important

Depends on NativePHP/mobile-air#241 — this consumes ChromeScrollController from core and will not compile without it. Merge and release that first.

1. auto_scroll_to was never implemented

ScrollView::autoScrollTo() sets the prop, but no renderer on either platform read it — it was written into the wire payload and dropped on the floor. BenchmarkComponent.php:992 relies on it to scroll a long list, so that benchmark scenario has been measuring a static list.

• iOS — reuses the ScrollViewReader already on the vertical branch, scrolling to the target child's id; the horizontal branch gains a reader so it behaves the same.
• Android — drives LazyColumn's existing listState by index; LazyRow gains a state to drive.

Both jump without animation on first appear (opening at a target shouldn't look like a scroll) and animate later moves, matching the existing scroll-anchor path. An explicit index wins over scroll-anchor="bottom". Negative and out-of-range indices are no-ops.

2. gap-* was ignored on Android

The flex engine lays out ordinary containers, but a lazy list arranges its own items — so gap was read on iOS (into LazyVStack/LazyHStack spacing) and silently dropped on Android.

That gap matters more than it looks. Without it the only way to space a list is wrapping it in a <column>, which collapses the entire list into a single lazy item — killing virtualization. Measured on a real chat log: directChildren=1, canScrollForward=false, and firstVisibleItemScrollOffset climbing past 1000px inside one item.

3. Programmatic scrolls left a collapsing top bar expanded

Both scroll paths now drive core's ChromeScrollController after moving the list. Skipped on the initial jump — opening a screen shouldn't animate the chrome shut.

Compatibility — please read

Two user-visible behaviour changes for existing apps:

• gap on a scroll-view now spaces on Android. Any app relying on the current no-op will see layouts shift. Arguably the fix, but it is a change.
• autoScrollTo() now scrolls. Anyone who called it and got nothing will now get scrolling.

Also auto_scroll_to taking precedence over scroll_anchor — previously moot, since auto_scroll_to did nothing.

Testing status — partial, please review accordingly

• Chrome collapse: verified on a physical Pixel 9. Bar travels 0 → -231, then short-circuits on repeat messages.
• auto_scroll_to (both platforms) and the Android gap fix: NOT yet exercised on device. The test app uses scroll-anchor="bottom" with a wrapped log, so neither path was hit. Reviewed against surrounding code, imports verified, but they want a real build before merge.

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