Route corner layout pages by region and harden page selection

The below-video panel now renders only when the info page is assigned
to that region; comments assigned below become the portrait panel's
pill (iOS), and every other below-video page falls back to the side
column instead of disappearing. Page selection snaps to the first
configured page when the current one is disabled or reordered away,
and the page bar highlight follows the actually rendered page. The
related tab stays visible with a spinner while video details load
instead of vanishing until they arrive.

Yattee/Views/Player/Panels/PlayerPanelContentView.swift

@@ -54,7 +54,11 @@ struct PlayerPanelContentView: View {
             return !video.isFromMediaSource
                 && playerState?.commentsState != .disabled
         case .related:
-            return !(video.relatedVideos ?? []).isEmpty
+            if !(video.relatedVideos ?? []).isEmpty { return true }
+            // Keep the tab while details are still loading (spinner in the
+            // page); hide only once details resolved with no related videos.
+            let detailsState = playerState?.videoDetailsState ?? .idle
+            return detailsState == .idle || detailsState == .loading
         case .chapters:
             return !(playerState?.chapters ?? []).isEmpty
         case .queue:
@@ -76,13 +80,11 @@ struct PlayerPanelContentView: View {
         }
     }
 
-    private var visiblePages: [PanelPageConfiguration] {
-        var pages = Self.visiblePages(
-            panelSettings: panelSettings,
-            region: region,
-            video: context.video,
-            appEnvironment: appEnvironment
-        )
+    /// Enabled pages for this host in configured order, before runtime
+    /// visibility filtering. The selection tracks this list so a page that
+    /// loads late (e.g. related) still wins once its content arrives.
+    private var configuredPages: [PanelPageConfiguration] {
+        var pages = panelSettings.enabledPages(in: region)
         if !excludedPages.isEmpty {
             pages = pages.filter { !excludedPages.contains($0.pageType) }
         }
@@ -90,6 +92,12 @@ struct PlayerPanelContentView: View {
         return pages.filter { $0.pageType != .chapters && $0.pageType != .related }
     }
 
+    private var visiblePages: [PanelPageConfiguration] {
+        configuredPages.filter {
+            Self.isRuntimeVisible($0.pageType, video: context.video, appEnvironment: appEnvironment)
+        }
+    }
+
     /// The page to render: the selection when visible, else the first page.
     private var effectivePage: PanelPageConfiguration? {
         visiblePages.first { $0.pageType == selectedPage } ?? visiblePages.first
@@ -102,9 +110,14 @@ struct PlayerPanelContentView: View {
 
         VStack(spacing: 0) {
             if showPageBar, pages.count >= 2 {
+                // Highlight follows the rendered page (the selection when
+                // visible, else the fallback); taps still set the selection.
                 PanelPageBar(
                     pages: pages,
-                    selectedPage: $selectedPage,
+                    selectedPage: Binding(
+                        get: { effectivePage?.pageType ?? selectedPage },
+                        set: { selectedPage = $0 }
+                    ),
                     accentColor: context.accentColor,
                     showLabels: panelSettings.effectiveShowPageBarLabels
                 )
@@ -118,14 +131,23 @@ struct PlayerPanelContentView: View {
                 Spacer()
             }
         }
+        .onChange(of: configuredPages.map(\.pageType), initial: true) { _, pageTypes in
+            // Snap a selection that isn't configured for this host (e.g. the
+            // host's default page was disabled or reordered away) to the
+            // first configured page.
+            if !pageTypes.contains(selectedPage), let first = pageTypes.first {
+                selectedPage = first
+            }
+        }
         .onChange(of: selectedPage) { _, _ in
             // Each page owns its scroll position; reset the host's offset so
             // pills and the scroll-to-top button start from the top state.
             scrollConfig.scrollOffsetChanged?(0)
         }
         .onChange(of: context.video.id) { _, _ in
-            // New video: fall back to the first configured page.
-            if let first = visiblePages.first?.pageType {
+            // New video: reset to the first configured page (not the runtime
+            // list — related/chapters may not have loaded yet).
+            if let first = configuredPages.first?.pageType {
                 selectedPage = first
             }
         }