fix: normalize ChainPosition ffi byte accounting

lib/bdk.dart

@@ -5615,9 +5615,17 @@ class FfiConverterChainPosition {
     final subview = Uint8List.view(buf.buffer, buf.offsetInBytes + 4);
     switch (index) {
       case 1:
-        return ConfirmedChainPosition.read(subview);
+        final lifted = ConfirmedChainPosition.read(subview);
+        return LiftRetVal<ChainPosition>(
+          lifted.value,
+          lifted.bytesRead - subview.offsetInBytes + 4,
+        );
       case 2:
-        return UnconfirmedChainPosition.read(subview);
+        final lifted = UnconfirmedChainPosition.read(subview);
+        return LiftRetVal<ChainPosition>(
+          lifted.value,
+          lifted.bytesRead - subview.offsetInBytes + 4,
+        );
       default:
         throw UniffiInternalError(
           UniffiInternalError.unexpectedEnumCase,
@@ -5635,7 +5643,7 @@ class FfiConverterChainPosition {
   }
 
   static int write(ChainPosition value, Uint8List buf) {
-    return value.write(buf);
+    return value.write(buf) - buf.offsetInBytes;
   }
 }
 

test/chain_position_converter_test.dart

@@ -0,0 +1,44 @@
+import "dart:typed_data";
+
+import "package:bdk_dart/bdk.dart";
+import "package:test/test.dart";
+
+void main() {
+  group("ChainPosition ffi converter", () {
+    test("read reports relative bytes from a non-zero buffer offset", () {
+      final chainPosition = UnconfirmedChainPosition(1234567890);
+      final encoded = Uint8List(chainPosition.allocationSize());
+      final encodedLength = FfiConverterChainPosition.write(
+        chainPosition,
+        encoded,
+      );
+
+      expect(encodedLength, encoded.length);
+
+      const prefixLength = 11;
+      final padded = Uint8List(prefixLength + encoded.length);
+      padded.setRange(prefixLength, padded.length, encoded);
+
+      final lifted = FfiConverterChainPosition.read(
+        Uint8List.view(padded.buffer, prefixLength),
+      );
+
+      final unconfirmed = lifted.value as UnconfirmedChainPosition;
+      expect(unconfirmed.timestamp, 1234567890);
+      expect(lifted.bytesRead, encoded.length);
+    });
+
+    test("write reports relative bytes from a non-zero buffer offset", () {
+      final chainPosition = UnconfirmedChainPosition(1234567890);
+      const prefixLength = 7;
+      final buffer = Uint8List(prefixLength + chainPosition.allocationSize());
+
+      final bytesWritten = FfiConverterChainPosition.write(
+        chainPosition,
+        Uint8List.view(buffer.buffer, prefixLength),
+      );
+
+      expect(bytesWritten, chainPosition.allocationSize());
+    });
+  });
+}