diff --git a/tests/gtest/avifincrtest_helpers.cc b/tests/gtest/avifincrtest_helpers.cc
index da26e09..4149b61 100644
--- a/tests/gtest/avifincrtest_helpers.cc
+++ b/tests/gtest/avifincrtest_helpers.cc
@@ -118,13 +118,17 @@
 struct PartialData {
   avifROData available;
   size_t full_size;
+
+  // Only used as nonpersistent input.
+  std::unique_ptr<uint8_t[]> nonpersistent_bytes;
+  size_t num_nonpersistent_bytes;
 };
 
 // Implementation of avifIOReadFunc simulating a stream from an array. See
 // avifIOReadFunc documentation. io->data is expected to point to PartialData.
 avifResult PartialRead(struct avifIO* io, uint32_t read_flags, uint64_t offset,
                        size_t size, avifROData* out) {
-  const PartialData* data = (PartialData*)io->data;
+  PartialData* data = reinterpret_cast<PartialData*>(io->data);
   if ((read_flags != 0) || !data || (data->full_size < offset)) {
     return AVIF_RESULT_IO_ERROR;
   }
@@ -134,7 +138,23 @@
   if (data->available.size < (offset + size)) {
     return AVIF_RESULT_WAITING_ON_IO;
   }
-  out->data = data->available.data + offset;
+  if (io->persistent) {
+    out->data = data->available.data + offset;
+  } else {
+    // Dedicated buffer containing just the available bytes and nothing more.
+    std::unique_ptr<uint8_t[]> bytes(new uint8_t[size]);
+    std::copy(data->available.data + offset,
+              data->available.data + offset + size, bytes.get());
+    out->data = bytes.get();
+    // Flip the previously returned bytes to make sure the values changed.
+    for (size_t i = 0; i < data->num_nonpersistent_bytes; ++i) {
+      data->nonpersistent_bytes[i] = ~data->nonpersistent_bytes[i];
+    }
+    // Free the memory to invalidate the old pointer. Only do that after
+    // allocating the new bytes to make sure to have a different pointer.
+    data->nonpersistent_bytes = std::move(bytes);
+    data->num_nonpersistent_bytes = size;
+  }
   out->size = size;
   return AVIF_RESULT_OK;
 }
@@ -258,8 +278,9 @@
   }
 
   // Emulate a byte-by-byte stream.
-  PartialData data = {/*available=*/{encoded_avif.data, 0},
-                      /*fullSize=*/encoded_avif.size};
+  PartialData data = {
+      /*available=*/{encoded_avif.data, 0}, /*fullSize=*/encoded_avif.size,
+      /*nonpersistent_bytes=*/nullptr, /*num_nonpersistent_bytes=*/0};
   avifIO io = {
       /*destroy=*/nullptr, PartialRead,
       /*write=*/nullptr,   give_size_hint ? encoded_avif.size : 0,
@@ -269,7 +290,7 @@
   ASSERT_NE(decoder, nullptr);
   avifDecoderSetIO(decoder.get(), &io);
   decoder->allowIncremental = AVIF_TRUE;
-  const size_t step = std::max(static_cast<size_t>(1), data.full_size / 10000);
+  const size_t step = std::max<size_t>(1, data.full_size / 10000);
 
   // Parsing is not incremental.
   avifResult parse_result = avifDecoderParse(decoder.get());