Summary

RCTViewComponentView.mm did not compile on macOS in 0.83-merge. The upstream 0.83 merge brought in new iOS-only methods that aren't wrapped in #if !TARGET_OS_OSX guards, leaving them visible to the macOS compile where they trip
duplicate-declaration errors against the existing AppKit-flavored methods in the macOS branch + use UIView / UIColor types unknown to AppKit.

Concrete errors blocking xcodebuild -scheme RNTester-macOS:

RCTViewComponentView.mm:810:3: error: unknown type name 'UIView'; did you mean 'NSView'?
RCTViewComponentView.mm:1048:4: error: expected a type (- (UIView )effectiveContentView)
RCTViewComponentView.mm:1371:11: error: unknown type name 'UIColor'; did you mean 'CIColor'?
RCTViewComponentView.mm:2412:43: error: expected a type (transferVisualPropertiesFromView:(UIView))
RCTViewComponentView.mm:2465:1: error: duplicate declaration of method 'handleCommand:args:'
RCTViewComponentView.mm:2478:1: error: duplicate declaration of method 'focus'
RCTViewComponentView.mm:2483:1: error: duplicate declaration of method 'blur'
RCTViewComponentView.mm:2490:1: error: duplicate declaration of method 'becomeFirstResponder'
RCTViewComponentView.mm:2503:1: error: duplicate declaration of method 'resignFirstResponder'

The macOS branch at lines 1846-2381 already provides AppKit-flavored versions of focus, blur, becomeFirstResponder, resignFirstResponder, handleCommand: — the duplicates after that block are iOS-canonical implementations
brought in by the upstream merge.

This PR is surgical: every change is either (a) wrapping iOS code in #if !TARGET_OS_OSX so it disappears from the macOS compile, or (b) swapping UIView* / UIColor* types for the existing fork-wide RCTPlatformView* /
RCTUIColor* compatibility aliases that resolve to the right AppKit class on macOS. No iOS behavior changes — every iOS code path executes identically before and after.

Changes (5 surgical edits)

1. Line 810 — hitTest: local var. UIView *currentContainerView → RCTPlatformView *currentContainerView. The getter (currentContainerView declared at 1107) already returns RCTUIView* which is NSView* on macOS and UIView*
   on iOS; the local just needed to match.

2. Lines 1048-1102 — effectiveContentView method. Return type UIView* → RCTPlatformView*. Wrapped the SwiftUI-filter body in #if TARGET_OS_OSX ... return self; #else ... <existing body> ... #endif. The SwiftUI-filter wrapping
   uses RCTSwiftUIContainerViewWrapper which currently assumes UIKit's UIView surface — not yet ported to macOS. macOS falls back to the trivial return self path (callers continue to work because the type is RCTPlatformView*).

3. Line 1109 — currentContainerView caller. UIView *effectiveContentView = self.effectiveContentView; → RCTPlatformView *effectiveContentView = .... Matches the updated return type from change Update readme #2.

4. Line 1371 — SwiftUI dropShadow color. UIColor *shadowColor = RCTUIColorFromSharedColor(...) → RCTUIColor *shadowColor = .... RCTUIColor is the existing @compatibility_alias defined at
   React/RCTUIKit/RCTUIKitCompat.h:25-29 (UIColor on iOS, NSColor on macOS). RCTUIColorFromSharedColor already returns NSColor on macOS, and RCTSwiftUIContainerViewWrapper.h exposes the updateDropShadow:...color: selector with both
   UIColor (iOS) and NSColor (macOS) overloads — the typed alias picks the right one per platform.

5. Lines 2412-2515 — wrap iOS-only methods in #if !TARGET_OS_OSX. Covers transferVisualPropertiesFromView:toView: (UIView-typed args), canBecomeFirstResponder (UIResponder semantics — macOS uses acceptsFirstResponder
   instead), and the duplicates of handleCommand:args: / focus / blur / becomeFirstResponder / resignFirstResponder that conflict with the AppKit-flavored versions in the macOS branch above.

Relationship to other in-flight PRs

• Depends on fix(fmt): patch base.h consteval gate via prepare_command for Xcode 26.x #2961 (fmt FMT_USE_CONSTEVAL=0) — without it, pod install+xcodebuild fails at Pods/fmt/src/format.cc before ever reaching this file on Xcode 26.x.
• Coexists with fix(macos): three Fabric focus regressions (blur, sendAccessibilityEvent, VirtualView) #2959 (three Fabric focus regressions) — that PR touches the macOS branch of this same file (lines 1886-1924) for focus behavior; this PR touches different line ranges for compile correctness. No merge conflict.

Together, #2961 + this PR + #2959 are the minimum set required for xcodebuild -scheme RNTester-macOS build to succeed on 0.83-merge for Xcode 26.x. Without this PR, the file is rejected at the type-check stage and the whole
React-RCTFabric-macOS target fails to compile.

Closes the macOS-compile half of the "Verify focus changes" + "RNTester validation" items on the Road to 0.83 tracking issue (#2901). With #2960 (pbxproj restore) + this PR + #2961, xcodebuild ... -scheme RNTester-macOS build
now completes with ** BUILD SUCCEEDED **.

Test Plan

• Before: xcodebuild -workspace packages/rn-tester/RNTesterPods.xcworkspace -scheme RNTester-macOS -configuration Debug -destination 'generic/platform=macOS' ARCHS=arm64 ONLY_ACTIVE_ARCH=YES CODE_SIGNING_ALLOWED=NO build fails at
  CompileC .../React-RCTFabric-macOS.build/.../RCTViewComponentView.o with the 9+ errors quoted above. Reproducible on a clean 0.83-merge checkout running Xcode 26.x.
• After: same xcodebuild produces ** BUILD SUCCEEDED ** and a runnable Build/Products/Debug/RNTester-macOS.app. Validated locally with Xcode 26.5 / arm64 / Debug after applying both fix(fmt): patch base.h consteval gate via prepare_command for Xcode 26.x #2961 and this PR.
• iOS regression check: every edit either swaps UIView/UIColor for RCTPlatformView/RCTUIColor (compat aliases that resolve identically to the original types on iOS) or moves iOS code inside #if !TARGET_OS_OSX. No iOS code
  path is removed; iOS RNTester build is unaffected.
• 311 warnings remain after this PR — all pre-existing on 0.83-merge (deprecation notices on AppKit APIs, "build phase runs every build" hints, etc.), unrelated to this change.

Related

• Road to 0.83 #2901 — Road to 0.83 tracking issue (closes the macOS-compile blocker for "Verify focus changes" / "RNTester validation")
• fix(fmt): patch base.h consteval gate via prepare_command for Xcode 26.x #2961 — fmt FMT_USE_CONSTEVAL=0 (prerequisite for reaching this file's compile on Xcode 26.x)
• fix(macos): three Fabric focus regressions (blur, sendAccessibilityEvent, VirtualView) #2959 — Three Fabric focus regressions (touches same file, different lines, no conflict)
• fix(rn-tester): restore RNTesterPods.xcodeproj/project.pbxproj on 0.83-merge #2960 — Restore project.pbxproj (prerequisite for any pod install on 0.83-merge)