diff --git a/media/gpu/windows/media_foundation_video_encode_accelerator_win.cc b/media/gpu/windows/media_foundation_video_encode_accelerator_win.cc
index 0000000..0000000 100644
--- a/media/gpu/windows/media_foundation_video_encode_accelerator_win.cc
+++ b/media/gpu/windows/media_foundation_video_encode_accelerator_win.cc
@@ -2390,9 +2390,18 @@ HRESULT MediaFoundationVideoEncodeAccelerator::PopulateInputSampleBufferGpu(
     hr = texture_buffer.As(&dxgi_buffer);
     RETURN_ON_HR_FAILURE(hr, "Failed to get dxgi buffer", hr);
     hr = dxgi_buffer->GetResource(IID_PPV_ARGS(&input_texture));
   }

-  if (frame->visible_rect().size() != input_visible_size_) {
+  // Mirror the texture-size check in CopyInputSampleBufferFromGpu().  The
+  // SharedImage zero-copy path (mf_helpers.cc CreateSampleFromTexture /
+  // GenerateResourceOnSyncTokenReleased) allocates a coded_size texture with
+  // pInitialData=nullptr and copies only |visible_rect| into it; without this
+  // check, the uninitialised tail (stale GPU VRAM) is handed to the encoder MFT
+  // and can leak into the output bitstream.
+  D3D11_TEXTURE2D_DESC input_desc = {};
+  input_texture->GetDesc(&input_desc);
+  gfx::Size texture_size(input_desc.Width, input_desc.Height);
+  if (texture_size != input_visible_size_ ||
+      frame->visible_rect().size() != input_visible_size_) {
     hr = PerformD3DScaling(input_texture.Get(), frame->visible_rect());
     RETURN_ON_HR_FAILURE(hr, "Failed to perform D3D video processing", hr);
     sample_texture = scaled_d3d11_texture_;
   } else if (input.generate_sample_on_wait_sync_token) {
diff --git a/media/base/win/mf_helpers.cc b/media/base/win/mf_helpers.cc
index 0000000..0000000 100644
--- a/media/base/win/mf_helpers.cc
+++ b/media/base/win/mf_helpers.cc
@@ -907,14 +907,13 @@ Microsoft::WRL::ComPtr<IMFSample> CreateSampleFromTexture(
                  << logging::SystemErrorCodeToString(hr);
       return nullptr;
     }
     Microsoft::WRL::ComPtr<ID3D11DeviceContext> device_context;
     device->GetImmediateContext(&device_context);
-    D3D11_BOX src_box = {static_cast<UINT>(frame->visible_rect().x()),
-                         static_cast<UINT>(frame->visible_rect().y()),
-                         0,
-                         static_cast<UINT>(frame->visible_rect().right()),
-                         static_cast<UINT>(frame->visible_rect().bottom()),
-                         1};
+    // Copy the entire source texture so the destination (allocated with
+    // pInitialData=nullptr, i.e. uninitialised default-heap GPU memory) has no
+    // unwritten tail that could leak stale VRAM into the encoder bitstream.
+    // The destination was created with the source's GetDesc(), so a full copy
+    // is always in-bounds.
     device_context->CopySubresourceRegion(copied_texture.Get(), 0, 0, 0, 0,
-                                          input_texture.Get(), 0, &src_box);
+                                          input_texture.Get(), 0, nullptr);
     input_texture = copied_texture;
   }
@@ -1176,15 +1175,12 @@ void GenerateResourceOnSyncTokenReleased(

     Microsoft::WRL::ComPtr<ID3D11DeviceContext> device_context;
     shared_d3d11_device->GetImmediateContext(&device_context);
-    D3D11_BOX src_box = {static_cast<UINT>(frame->visible_rect().x()),
-                         static_cast<UINT>(frame->visible_rect().y()),
-                         0,
-                         static_cast<UINT>(frame->visible_rect().right()),
-                         static_cast<UINT>(frame->visible_rect().bottom()),
-                         1};
+    // Copy the full subresource so the freshly-allocated (uninitialised)
+    // |shared_texture| has no unwritten tail.  See CreateSampleFromTexture()
+    // for rationale.
     device_context->CopySubresourceRegion(shared_texture.Get(), 0, 0, 0, 0,
                                           input_texture.texture.Get(),
-                                          input_texture.array_index, &src_box);
+                                          input_texture.array_index, nullptr);
     Microsoft::WRL::ComPtr<IDXGIResource1> shared_dxgi_resource;
     hr = shared_texture.As(&shared_dxgi_resource);
     CHECK_EQ(hr, S_OK);