fix: (flutter) Linux OpenGL external texture orientation

thermion_flutter/thermion_flutter/lib/src/platform/src/method_channel_platform_texture_descriptor.dart

@@ -12,6 +12,7 @@ class MethodChannelPlatformTextureDescriptor extends PlatformTextureDescriptor {
     required super.windowHandle,
     required super.width,
     required super.height,
+    super.flipVertically,
     bool deferred = false,
   }) : _deferred = deferred;
 
@@ -38,6 +39,7 @@ class MethodChannelPlatformTextureDescriptor extends PlatformTextureDescriptor {
     int width,
     int height, {
     bool deferred = false,
+    bool flipVertically = false,
   }) async {
     final allocation = await allocateMethodChannelTexture(
       channel,
@@ -52,6 +54,7 @@ class MethodChannelPlatformTextureDescriptor extends PlatformTextureDescriptor {
       width: width,
       height: height,
       deferred: deferred,
+      flipVertically: flipVertically,
     );
   }
 

thermion_flutter/thermion_flutter/lib/src/platform/src/native_texture_surface_manager.dart

@@ -62,6 +62,7 @@ class NativeTextureSurfaceManager {
          resumeRenderingIfReady: lifecycle.resumeRenderingIfReady,
          runTextureMutation: lifecycle.duringTextureMutation,
          androidTextureSource: () => options().androidTextureSource,
+         flipLinuxTextureVertically: () => options().backend != Backend.VULKAN,
        );
 
   static final _logger = Logger('NativeTextureSurfaceManager');

thermion_flutter/thermion_flutter/lib/src/platform/src/platform_texture_descriptor.dart

@@ -11,6 +11,12 @@ abstract class PlatformTextureDescriptor {
 
   int height;
 
+  /// Whether Flutter must invert this texture vertically when presenting it.
+  ///
+  /// OpenGL render targets use a bottom-left origin, while Flutter's widget
+  /// coordinate system uses a top-left origin.
+  final bool flipVertically;
+
   View? _boundView;
 
   /// The (possibly null) view this descriptor is bound to
@@ -28,6 +34,7 @@ abstract class PlatformTextureDescriptor {
     required this.width,
     required this.height,
     this.windowHandle,
+    this.flipVertically = false,
   });
 
   /// Descriptors are identified by their Flutter texture id. This is stable

thermion_flutter/thermion_flutter/lib/src/platform/src/platform_texture_descriptor_registry_native.dart

@@ -39,12 +39,17 @@ class NativePlatformTextureDescriptorRegistry
     required void Function() resumeRenderingIfReady,
     required TextureMutationRunner runTextureMutation,
     required AndroidTextureSource Function() androidTextureSource,
+    required bool Function() flipLinuxTextureVertically,
   }) : _pauseRendering = pauseRendering,
        _resumeRenderingIfReady = resumeRenderingIfReady,
        _runTextureMutation = runTextureMutation,
        super(
-         allocator: (width, height) =>
-             _allocate(width, height, androidTextureSource()),
+         allocator: (width, height) => _allocate(
+           width,
+           height,
+           androidTextureSource(),
+           flipLinuxTextureVertically(),
+         ),
        ) {
     if (Platform.isAndroid) {
       channel.setMethodCallHandler(_handlePlatformMethodCall);
@@ -62,6 +67,7 @@ class NativePlatformTextureDescriptorRegistry
     int width,
     int height,
     AndroidTextureSource androidTextureSource,
+    bool flipLinuxTextureVertically,
   ) {
     if (Platform.isMacOS || Platform.isIOS) {
       return Future.value(
@@ -89,6 +95,7 @@ class NativePlatformTextureDescriptorRegistry
         width,
         height,
         deferred: true,
+        flipVertically: flipLinuxTextureVertically,
       );
     }
     throw UnsupportedError('Platform textures are not supported on $Platform');

thermion_flutter/thermion_flutter/lib/src/widgets/src/thermion_widget_internal/texture_widget_builder.dart

@@ -7,10 +7,17 @@ Widget surfaceWidgetBuilder(PlatformTextureDescriptor? descriptor, View view) {
   if (descriptor == null) {
     return const SizedBox.shrink();
   }
-  return Texture(
+  return buildTextureWidget(descriptor);
+}
+
+@visibleForTesting
+Widget buildTextureWidget(PlatformTextureDescriptor descriptor) {
+  final texture = Texture(
     key: ObjectKey("flutter_texture_${descriptor.flutterTextureId}"),
     textureId: descriptor.flutterTextureId,
     filterQuality: FilterQuality.none,
     freeze: false,
   );
+  if (!descriptor.flipVertically) return texture;
+  return Transform.flip(flipY: true, child: texture);
 }

thermion_flutter/thermion_flutter/test/thermion_widget_resize_transition_test.dart

@@ -1,6 +1,8 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_test/flutter_test.dart';
+import 'package:thermion_flutter/src/platform/src/platform_texture_descriptor.dart';
 import 'package:thermion_flutter/src/widgets/src/thermion_widget.dart';
+import 'package:thermion_flutter/src/widgets/src/thermion_widget_internal/texture_widget_builder.dart';
 
 void main() {
   testWidgets('staged replacement is mounted below the last valid frame', (
@@ -48,4 +50,45 @@ void main() {
     expect(find.byKey(currentKey), findsOneWidget);
     expect(find.byType(Stack), findsNothing);
   });
+
+  testWidgets('flagged platform texture is vertically flipped', (tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: buildTextureWidget(_TextureDescriptor(flipVertically: true)),
+      ),
+    );
+
+    final transform = tester.widget<Transform>(find.byType(Transform));
+    expect(transform.transform.entry(0, 0), 1);
+    expect(transform.transform.entry(1, 1), -1);
+    expect(find.byType(Texture), findsOneWidget);
+  });
+
+  testWidgets('unflagged platform texture is not transformed', (tester) async {
+    await tester.pumpWidget(
+      MaterialApp(
+        home: buildTextureWidget(_TextureDescriptor(flipVertically: false)),
+      ),
+    );
+
+    expect(find.byType(Transform), findsNothing);
+    expect(find.byType(Texture), findsOneWidget);
+  });
+}
+
+class _TextureDescriptor extends PlatformTextureDescriptor {
+  _TextureDescriptor({required bool flipVertically})
+    : super(
+        flutterTextureId: 1,
+        hardwareId: 2,
+        width: 3,
+        height: 4,
+        flipVertically: flipVertically,
+      );
+
+  @override
+  Future<void> destroy() async {}
+
+  @override
+  Future<void> markTextureFrameAvailable() async {}
 }