fix(ios): reset EmptyLayoutMetrics on Fabric view recycle

[kosmydel]

Summary:

On iOS Fabric, a recycled RCTViewComponentView can keep UIView.hidden = YES after reuse, so remounted visible content stays blank (React mounted, native host invisible).

On default stock iOS, this usually does not show up in normal RN usage. The mount slicer skips Trait::Hidden (display: 'none'), so those nodes are torn down rather than left as hosts with UIView.hidden = YES, and the recycle pool is not poisoned that way.

The bug is still real whenever a host does enter the pool with hidden=YES and is later reused for visible Flex content. On reuse, RCTViewComponentView ignores the mounting manager’s oldLayoutMetrics argument and passes its stored _layoutMetrics instead. After prepareForRecycle that stored value is zero-init Flex ({}), not EmptyLayoutMetrics, so Flex→Flex skips updating hidden. React still mounts/layouts; the native view stays blank.

That can happen in apps (happens in one of our biggest client's app) that keep display: 'none' mounted and hide via UIView.hidden, or any other path that sets hidden before unmount. Once poisoned, default View recycling reproduces blank UI. Confirmed workaround: +shouldBeRecycled { return NO; } (mitigation only).

This is recycle hygiene in the same family as other prepareForRecycle leaks (e.g. ScrollView inset/offset). The Insert path already treats EmptyLayoutMetrics as “apply all layout-derived UIView state”; recycle should leave that same sentinel so remounts stay correct when recycling is used.

RCTViewComponentView prepareForRecycle currently does:

_layoutMetrics = {};

That is not EmptyLayoutMetrics (frame -1×-1). UIView+ComponentViewProtocol updateLayoutMetrics only force-updates UIView.hidden when oldLayoutMetrics == EmptyLayoutMetrics:

bool forceUpdate = oldLayoutMetrics == EmptyLayoutMetrics;
if (forceUpdate || (layoutMetrics.displayType != oldLayoutMetrics.displayType)) {
  self.hidden = layoutMetrics.displayType == DisplayType::None;
}

Fix: assign EmptyLayoutMetrics in prepareForRecycle so the next mount force-updates hidden (and other layout-gated UIView state) correctly. One-line change; no API surface change.

Introduced in: #55796

Minimal repro: https://github.com/kosmydel/rn-fabric-recycle-hidden

Changelog:

[iOS] [Fixed] - Reset EmptyLayoutMetrics in RCTViewComponentView prepareForRecycle so recycled views clear UIView.hidden

Test Plan:

1. Clone https://github.com/kosmydel/rn-fabric-recycle-hidden (RN 0.86, New Arch / iOS).
2. npm install && cd ios && bundle exec pod install && cd .. && npm run ios
3. Tap Run (or wait for auto-run).

Before this change (shouldBeRecycled = YES, default):

• Expected: red square visible after remount
• Actual: gray empty slot; status FAIL — recycled view still hidden=YES (1/1)

With +shouldBeRecycled { return NO; } (workaround):

• Red square visible (PASS) — confirms failure is recycle reuse, not JS unmount

After this change (rebuild RN from source / with the patch):

• Same repro shows red square + PASS with recycling still enabled

[meta-codesync]

@javache has imported this pull request. If you are a Meta employee, you can view this in D112528840.