improved unknown file preview handling with probe failure fallbacks and switched to an explicit TabController in the share view to prevent build-time layout issues

lib/view/pages/more/share/qr_share_view.dart

@@ -17,7 +17,19 @@ class QrShareView extends StatefulWidget {
   State<QrShareView> createState() => _QrShareViewState();
 }
 
-class _QrShareViewState extends State<QrShareView> {
+class _QrShareViewState extends State<QrShareView>
+    with SingleTickerProviderStateMixin {
+  // Owning the TabController explicitly (instead of DefaultTabController) is
+  // a workaround for a Flutter framework bug where TabBarView's
+  // didChangeDependencies issues a jumpToPage mid-build, whose scroll
+  // notification then calls setState on _TabStyle while the frame is still
+  // being built — only reproduces reliably with a bottomNavigationBar in the
+  // Scaffold underneath.
+  late final TabController _tabController = TabController(
+    length: 2,
+    vsync: this,
+  );
+
   @override
   void initState() {
     ScreenBrightness.instance.setApplicationScreenBrightness(1.0);
@@ -26,6 +38,7 @@ class _QrShareViewState extends State<QrShareView> {
 
   @override
   void dispose() {
+    _tabController.dispose();
     ScreenBrightness.instance.resetApplicationScreenBrightness();
     super.dispose();
   }
@@ -45,53 +58,52 @@ class _QrShareViewState extends State<QrShareView> {
   }
 
   @override
-  Widget build(BuildContext context) => DefaultTabController(
-    length: 2,
-    child: Scaffold(
-      appBar: AppBar(
-        title: const Text('Teile die App'),
-        bottom: const TabBar(
-          tabs: [
-            Tab(icon: Icon(Icons.android_outlined), text: 'Android'),
-            Tab(icon: Icon(Icons.apple_outlined), text: 'iOS & iPadOS'),
-          ],
-        ),
-      ),
-      body: const TabBarView(
-        children: [
-          AppSharePlatformView('Für Android', _androidUrl),
-          AppSharePlatformView('Für iOS & iPad', _iosUrl),
+  Widget build(BuildContext context) => Scaffold(
+    appBar: AppBar(
+      title: const Text('Teile die App'),
+      bottom: TabBar(
+        controller: _tabController,
+        tabs: const [
+          Tab(icon: Icon(Icons.android_outlined), text: 'Android'),
+          Tab(icon: Icon(Icons.apple_outlined), text: 'iOS & iPadOS'),
         ],
       ),
-      bottomNavigationBar: SafeArea(
-        child: Material(
-          color: Theme.of(context).colorScheme.surface,
-          child: Column(
-            mainAxisSize: MainAxisSize.min,
-            children: [
-              const Divider(height: 1),
-              Builder(
-                builder: (innerCtx) => ListTile(
-                  leading: const Icon(Icons.share_outlined),
-                  title: const Text('Per Link teilen'),
-                  trailing: const Icon(Icons.arrow_right),
-                  onTap: () => _shareLink(innerCtx),
-                ),
+    ),
+    body: TabBarView(
+      controller: _tabController,
+      children: const [
+        AppSharePlatformView('Für Android', _androidUrl),
+        AppSharePlatformView('Für iOS & iPad', _iosUrl),
+      ],
+    ),
+    bottomNavigationBar: SafeArea(
+      child: Material(
+        color: Theme.of(context).colorScheme.surface,
+        child: Column(
+          mainAxisSize: MainAxisSize.min,
+          children: [
+            const Divider(height: 1),
+            Builder(
+              builder: (innerCtx) => ListTile(
+                leading: const Icon(Icons.share_outlined),
+                title: const Text('Per Link teilen'),
+                trailing: const Icon(Icons.arrow_right),
+                onTap: () => _shareLink(innerCtx),
               ),
-              ListTile(
-                leading: const Icon(Icons.android_outlined),
-                title: const Text('Android-Link kopieren'),
-                trailing: const Icon(Icons.copy),
-                onTap: () => copyToClipboard(context, _androidUrl),
-              ),
-              ListTile(
-                leading: const Icon(Icons.apple_outlined),
-                title: const Text('iOS-Link kopieren'),
-                trailing: const Icon(Icons.copy),
-                onTap: () => copyToClipboard(context, _iosUrl),
-              ),
-            ],
-          ),
+            ),
+            ListTile(
+              leading: const Icon(Icons.android_outlined),
+              title: const Text('Android-Link kopieren'),
+              trailing: const Icon(Icons.copy),
+              onTap: () => copyToClipboard(context, _androidUrl),
+            ),
+            ListTile(
+              leading: const Icon(Icons.apple_outlined),
+              title: const Text('iOS-Link kopieren'),
+              trailing: const Icon(Icons.copy),
+              onTap: () => copyToClipboard(context, _iosUrl),
+            ),
+          ],
         ),
       ),
     ),

lib/widget/file_viewer.dart

@@ -508,8 +508,6 @@ class _FileViewerState extends State<FileViewer> {
   Widget _buildUnknownPlaceholder() {
     final theme = Theme.of(context);
     final descriptors = _availableActions();
-    final remote = widget.remoteFile;
-    final hasPreview = remote?.hasPreview == true;
     return ListView(
       padding: const EdgeInsets.symmetric(vertical: 24),
       children: [
@@ -517,16 +515,8 @@ class _FileViewerState extends State<FileViewer> {
           padding: const EdgeInsets.symmetric(horizontal: 24),
           child: Column(
             children: [
-              _UnknownPreviewHeader(remoteFile: remote),
+              _UnknownPreviewBlock(remoteFile: widget.remoteFile),
               const SizedBox(height: 16),
-              if (!hasPreview) ...[
-                Text(
-                  'Vorschau nicht verfügbar',
-                  style: theme.textTheme.titleMedium,
-                  textAlign: TextAlign.center,
-                ),
-                const SizedBox(height: 6),
-              ],
               Text(
                 widget.path.split('/').last,
                 style: theme.textTheme.titleSmall?.copyWith(
@@ -607,24 +597,48 @@ class _TextPayload {
   const _TextPayload({required this.content, required this.truncated});
 }
 
-/// Header for the "Vorschau nicht verfügbar" screen: tries to fetch the
-/// Nextcloud thumbnail and shows it mid-sized if available, otherwise
-/// falls back to the generic file icon.
-class _UnknownPreviewHeader extends StatelessWidget {
+/// Header block for the "Vorschau nicht verfügbar" screen.
+///
+/// Two visual modes — kept layout-equivalent so the screen looks identical
+/// whether the server already said "no preview" or the probe failed late:
+///   * **No preview available** (server said no, no remoteFile, or probe
+///     errored): compact "file icon + 'Vorschau nicht verfügbar' text".
+///   * **Preview rendering / loaded**: mid-sized thumbnail without text.
+class _UnknownPreviewBlock extends StatefulWidget {
   final RemoteFileRef? remoteFile;
-  const _UnknownPreviewHeader({required this.remoteFile});
+  const _UnknownPreviewBlock({required this.remoteFile});
 
+  @override
+  State<_UnknownPreviewBlock> createState() => _UnknownPreviewBlockState();
+}
+
+class _UnknownPreviewBlockState extends State<_UnknownPreviewBlock> {
   static const double _previewSize = 180;
+  bool _failed = false;
 
-  Widget _fallbackIcon() =>
-      const Icon(Icons.insert_drive_file_outlined, size: 60);
+  Widget _compact(ThemeData theme) => Column(
+    mainAxisSize: MainAxisSize.min,
+    children: [
+      const Icon(Icons.insert_drive_file_outlined, size: 60),
+      const SizedBox(height: 16),
+      Text(
+        'Vorschau nicht verfügbar',
+        style: theme.textTheme.titleMedium,
+        textAlign: TextAlign.center,
+      ),
+    ],
+  );
 
   @override
   Widget build(BuildContext context) {
-    final remote = remoteFile;
-    // Skip the probe outright when the server already told us there is no
-    // preview — saves an HTTP request that would 404 anyway.
-    if (remote == null || remote.hasPreview == false) return _fallbackIcon();
+    final theme = Theme.of(context);
+    final remote = widget.remoteFile;
+    final canProbe =
+        remote != null &&
+        remote.hasPreview != false &&
+        remote.fileId != null &&
+        !_failed;
+    if (!canProbe) return _compact(theme);
     return SizedBox(
       width: _previewSize,
       height: _previewSize,
@@ -633,10 +647,19 @@ class _UnknownPreviewHeader extends StatelessWidget {
         imageUrl: _ncPreviewUrl(remote, width: 360),
         fadeInDuration: Duration.zero,
         fadeOutDuration: Duration.zero,
-        errorListener: (_) {},
+        // Late probe failure: re-render into the compact layout so the
+        // screen doesn't keep a 180×180 box around a tiny icon. Deferred
+        // to the next frame because setState during build is illegal.
+        errorListener: (_) {
+          if (!mounted) return;
+          WidgetsBinding.instance.addPostFrameCallback((_) {
+            if (mounted) setState(() => _failed = true);
+          });
+        },
         placeholder: (_, _) =>
             const Center(child: AppProgressIndicator.large()),
-        errorWidget: (_, _, _) => Center(child: _fallbackIcon()),
+        // Briefly empty while the post-frame setState swaps layouts.
+        errorWidget: (_, _, _) => const SizedBox.shrink(),
         imageBuilder: (_, imageProvider) => ClipRRect(
           borderRadius: BorderRadius.circular(8),
           child: Image(