fix: platform architecture picker visibility

Xcodes/Frontend/InfoPane/PlatformsView.swift

@@ -18,27 +18,30 @@ struct PlatformsView: View {
     var body: some View {
 
         let builds = xcode.sdks?.allBuilds
-        let runtimes = (builds?.flatMap { sdkBuild in
+        let availableRuntimes = (builds?.flatMap { sdkBuild in
             appState.downloadableRuntimes.filter {
-                $0.sdkBuildUpdate?.contains(sdkBuild) ?? false &&
-                ($0.architectures?.isEmpty ?? true ||
-                 ($0.architectures?.isUniversal ?? false && selectedVariant == .universal) ||
-                 ($0.architectures?.isAppleSilicon ?? false && selectedVariant == .appleSilicon)
-                )
+                $0.sdkBuildUpdate?.contains(sdkBuild) ?? false
             }
         } ?? []).removingReleaseCandidateDisplayDuplicates(installedRuntimes: appState.installedRuntimes)
-
-        let architectures = Set(runtimes.flatMap { $0.architectures ?? [] })
+
+        let availableVariants = ArchitectureVariant.allCases.filter { variant in
+            availableRuntimes.contains { $0.supports(variant) }
+        }
+        let displayedVariant = availableVariants.count == 1 ? availableVariants.first : selectedVariant
+        let runtimes = availableRuntimes.filter { runtime in
+            guard !(runtime.architectures?.isEmpty ?? true), let displayedVariant else { return true }
+            return runtime.supports(displayedVariant)
+        }
 
         VStack {
             HStack {
                 Text("Platforms")
                     .font(.title3)
                     .frame(maxWidth: .infinity, alignment: .leading)
-                if !architectures.isEmpty {
+                if availableVariants.count > 1 {
                     Spacer()
                     Picker("Architecture", selection: $selectedVariant) {
-                        ForEach(ArchitectureVariant.allCases, id: \.self) { arch in
+                        ForEach(availableVariants, id: \.self) { arch in
                             Label(variantLabel(for: arch), systemImage: arch.iconName)
                                 .tag(arch)
                         }
@@ -138,6 +141,15 @@ private struct RuntimeDisplayKey: Hashable {
 }
 
 private extension DownloadableRuntime {
+    func supports(_ variant: ArchitectureVariant) -> Bool {
+        switch variant {
+        case .universal:
+            return architectures?.isUniversal ?? false
+        case .appleSilicon:
+            return architectures?.isAppleSilicon ?? false
+        }
+    }
+
     var isReleaseCandidate: Bool {
         name.localizedCaseInsensitiveContains("Release Candidate") ||
         identifier.localizedCaseInsensitiveContains("_rc")