diff --git a/services/surfaceflinger/tests/unittests/Android.bp b/services/surfaceflinger/tests/unittests/Android.bp
index 40a6fb8..8269fcb 100644
--- a/services/surfaceflinger/tests/unittests/Android.bp
+++ b/services/surfaceflinger/tests/unittests/Android.bp
@@ -22,15 +22,40 @@
     default_team: "trendy_team_android_core_graphics_stack",
 }
 
+// This is a step towards pulling out the "backend" sources to clean up the
+// dependency graph between CompositionEngine and SurfaceFlinger.
+// MockNativeWindow doesn't strictly belong here, but this works for now so
+// that CompositionEngine tests can use these mocks.
 filegroup {
-    name: "libsurfaceflinger_mock_sources",
+    name: "libsurfaceflinger_backend_mock_sources",
     srcs: [
-        "mock/DisplayHardware/MockPowerHalController.cpp",
         "mock/DisplayHardware/MockComposer.cpp",
         "mock/DisplayHardware/MockHWC2.cpp",
+        "mock/DisplayHardware/MockHWComposer.cpp",
         "mock/DisplayHardware/MockIPower.cpp",
         "mock/DisplayHardware/MockPowerHintSessionWrapper.cpp",
         "mock/DisplayHardware/MockPowerAdvisor.cpp",
+        "mock/DisplayHardware/MockPowerHalController.cpp",
+        "mock/system/window/MockNativeWindow.cpp",
+    ],
+}
+
+cc_library_headers {
+    name: "libsurfaceflinger_backend_mock_headers",
+    export_include_dirs: ["."],
+    static_libs: [
+        "libgmock",
+        "libgtest",
+    ],
+    export_static_lib_headers: [
+        "libgmock",
+        "libgtest",
+    ],
+}
+
+filegroup {
+    name: "libsurfaceflinger_mock_sources",
+    srcs: [
         "mock/MockEventThread.cpp",
         "mock/MockFrameTimeline.cpp",
         "mock/MockFrameTracer.cpp",
@@ -39,7 +64,6 @@
         "mock/MockVsyncController.cpp",
         "mock/MockVSyncDispatch.cpp",
         "mock/MockVSyncTracker.cpp",
-        "mock/system/window/MockNativeWindow.cpp",
     ],
 }
 
@@ -60,6 +84,7 @@
     static_libs: ["libc++fs"],
     header_libs: ["surfaceflinger_tests_common_headers"],
     srcs: [
+        ":libsurfaceflinger_backend_mock_sources",
         ":libsurfaceflinger_mock_sources",
         ":libsurfaceflinger_sources",
         "libsurfaceflinger_unittest_main.cpp",
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWC2.h b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWC2.h
index 384b908..121104d 100644
--- a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWC2.h
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWC2.h
@@ -126,6 +126,8 @@
                 (uint32_t, const android::sp<android::GraphicBuffer> &,
                  const android::sp<android::Fence> &),
                 (override));
+    MOCK_METHOD(hal::Error, setBufferSlotsToClear,
+                (const std::vector<uint32_t>& slotsToClear, uint32_t activeBufferSlot), (override));
     MOCK_METHOD(hal::Error, setSurfaceDamage, (const android::Region &), (override));
     MOCK_METHOD(hal::Error, setBlendMode, (hal::BlendMode), (override));
     MOCK_METHOD(hal::Error, setColor, (aidl::android::hardware::graphics::composer3::Color),
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWComposer.cpp b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWComposer.cpp
new file mode 100644
index 0000000..f310633
--- /dev/null
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWComposer.cpp
@@ -0,0 +1,26 @@
+/*
+ * Copyright 2018 The Android Open Source Project
+
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "MockHWComposer.h"
+
+namespace android::mock {
+
+// The Google Mock documentation recommends explicit non-header instantiations
+// for better compile time performance.
+HWComposer::HWComposer() = default;
+HWComposer::~HWComposer() = default;
+
+} // namespace android::mock
diff --git a/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWComposer.h b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWComposer.h
new file mode 100644
index 0000000..fa7128c
--- /dev/null
+++ b/services/surfaceflinger/tests/unittests/mock/DisplayHardware/MockHWComposer.h
@@ -0,0 +1,153 @@
+/*
+ * Copyright 2024 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <gmock/gmock.h>
+
+#include "DisplayHardware/HWComposer.h"
+
+namespace android::mock {
+
+class HWComposer : public android::HWComposer {
+public:
+    using HWDisplayId = android::hardware::graphics::composer::hal::HWDisplayId;
+    using PowerMode = android::hardware::graphics::composer::hal::PowerMode;
+
+    HWComposer();
+    ~HWComposer() override;
+
+    MOCK_METHOD(void, setCallback, (HWC2::ComposerCallback&), (override));
+    MOCK_METHOD(bool, getDisplayIdentificationData,
+                (HWDisplayId, uint8_t*, DisplayIdentificationData*), (const, override));
+    MOCK_METHOD(bool, hasCapability, (aidl::android::hardware::graphics::composer3::Capability),
+                (const, override));
+    MOCK_METHOD(bool, hasDisplayCapability,
+                (HalDisplayId, aidl::android::hardware::graphics::composer3::DisplayCapability),
+                (const, override));
+
+    MOCK_METHOD(size_t, getMaxVirtualDisplayCount, (), (const, override));
+    MOCK_METHOD(size_t, getMaxVirtualDisplayDimension, (), (const, override));
+    MOCK_METHOD(bool, allocateVirtualDisplay, (HalVirtualDisplayId, ui::Size, ui::PixelFormat*),
+                (override));
+    MOCK_METHOD(void, allocatePhysicalDisplay,
+                (hal::HWDisplayId, PhysicalDisplayId, std::optional<ui::Size>), (override));
+
+    MOCK_METHOD(std::shared_ptr<HWC2::Layer>, createLayer, (HalDisplayId), (override));
+    MOCK_METHOD(status_t, getDeviceCompositionChanges,
+                (HalDisplayId, bool, std::optional<std::chrono::steady_clock::time_point>, nsecs_t,
+                 Fps, std::optional<android::HWComposer::DeviceRequestedChanges>*));
+    MOCK_METHOD(status_t, setClientTarget,
+                (HalDisplayId, uint32_t, const sp<Fence>&, const sp<GraphicBuffer>&, ui::Dataspace,
+                 float),
+                (override));
+    MOCK_METHOD(status_t, presentAndGetReleaseFences,
+                (HalDisplayId, std::optional<std::chrono::steady_clock::time_point>), (override));
+    MOCK_METHOD(status_t, executeCommands, (HalDisplayId));
+    MOCK_METHOD(status_t, setPowerMode, (PhysicalDisplayId, PowerMode), (override));
+    MOCK_METHOD(status_t, setColorTransform, (HalDisplayId, const mat4&), (override));
+    MOCK_METHOD(void, disconnectDisplay, (HalDisplayId), (override));
+    MOCK_METHOD(sp<Fence>, getPresentFence, (HalDisplayId), (const, override));
+    MOCK_METHOD(nsecs_t, getPresentTimestamp, (PhysicalDisplayId), (const, override));
+    MOCK_METHOD(sp<Fence>, getLayerReleaseFence, (HalDisplayId, HWC2::Layer*), (const, override));
+    MOCK_METHOD(status_t, setOutputBuffer,
+                (HalVirtualDisplayId, const sp<Fence>&, const sp<GraphicBuffer>&), (override));
+    MOCK_METHOD(void, clearReleaseFences, (HalDisplayId), (override));
+    MOCK_METHOD(status_t, getHdrCapabilities, (HalDisplayId, HdrCapabilities*), (override));
+    MOCK_METHOD(int32_t, getSupportedPerFrameMetadata, (HalDisplayId), (const, override));
+    MOCK_METHOD(std::vector<ui::RenderIntent>, getRenderIntents, (HalDisplayId, ui::ColorMode),
+                (const, override));
+    MOCK_METHOD(mat4, getDataspaceSaturationMatrix, (HalDisplayId, ui::Dataspace), (override));
+    MOCK_METHOD(status_t, getDisplayedContentSamplingAttributes,
+                (HalDisplayId, ui::PixelFormat*, ui::Dataspace*, uint8_t*), (override));
+    MOCK_METHOD(status_t, setDisplayContentSamplingEnabled, (HalDisplayId, bool, uint8_t, uint64_t),
+                (override));
+    MOCK_METHOD(status_t, getDisplayedContentSample,
+                (HalDisplayId, uint64_t, uint64_t, DisplayedFrameStats*), (override));
+    MOCK_METHOD(ftl::Future<status_t>, setDisplayBrightness,
+                (PhysicalDisplayId, float, float, const Hwc2::Composer::DisplayBrightnessOptions&),
+                (override));
+    MOCK_METHOD(std::optional<DisplayIdentificationInfo>, onHotplug,
+                (hal::HWDisplayId, hal::Connection), (override));
+    MOCK_METHOD(bool, updatesDeviceProductInfoOnHotplugReconnect, (), (const, override));
+    MOCK_METHOD(std::optional<PhysicalDisplayId>, onVsync, (hal::HWDisplayId, int64_t));
+    MOCK_METHOD(void, setVsyncEnabled, (PhysicalDisplayId, hal::Vsync), (override));
+    MOCK_METHOD(bool, isConnected, (PhysicalDisplayId), (const, override));
+    MOCK_METHOD(std::vector<HWComposer::HWCDisplayMode>, getModes, (PhysicalDisplayId, int32_t),
+                (const, override));
+    MOCK_METHOD((ftl::Expected<hal::HWConfigId, status_t>), getActiveMode, (PhysicalDisplayId),
+                (const, override));
+    MOCK_METHOD(std::vector<ui::ColorMode>, getColorModes, (PhysicalDisplayId), (const, override));
+    MOCK_METHOD(status_t, setActiveColorMode, (PhysicalDisplayId, ui::ColorMode, ui::RenderIntent),
+                (override));
+    MOCK_METHOD(ui::DisplayConnectionType, getDisplayConnectionType, (PhysicalDisplayId),
+                (const, override));
+    MOCK_METHOD(bool, isVsyncPeriodSwitchSupported, (PhysicalDisplayId), (const, override));
+    MOCK_METHOD((ftl::Expected<nsecs_t, status_t>), getDisplayVsyncPeriod, (PhysicalDisplayId),
+                (const, override));
+    MOCK_METHOD(status_t, setActiveModeWithConstraints,
+                (PhysicalDisplayId, hal::HWConfigId, const hal::VsyncPeriodChangeConstraints&,
+                 hal::VsyncPeriodChangeTimeline*),
+                (override));
+    MOCK_METHOD(status_t, setBootDisplayMode, (PhysicalDisplayId, hal::HWConfigId), (override));
+    MOCK_METHOD(status_t, clearBootDisplayMode, (PhysicalDisplayId), (override));
+    MOCK_METHOD(std::optional<hal::HWConfigId>, getPreferredBootDisplayMode, (PhysicalDisplayId),
+                (override));
+
+    MOCK_METHOD(std::vector<aidl::android::hardware::graphics::common::HdrConversionCapability>,
+                getHdrConversionCapabilities, (), (const, override));
+    MOCK_METHOD(status_t, setHdrConversionStrategy,
+                (aidl::android::hardware::graphics::common::HdrConversionStrategy,
+                 aidl::android::hardware::graphics::common::Hdr*),
+                (override));
+    MOCK_METHOD(status_t, setAutoLowLatencyMode, (PhysicalDisplayId, bool), (override));
+    MOCK_METHOD(status_t, getSupportedContentTypes,
+                (PhysicalDisplayId, std::vector<hal::ContentType>*), (const, override));
+    MOCK_METHOD(status_t, setContentType, (PhysicalDisplayId, hal::ContentType)), (override);
+    MOCK_METHOD((const std::unordered_map<std::string, bool>&), getSupportedLayerGenericMetadata,
+                (), (const, override));
+    MOCK_METHOD(void, dump, (std::string&), (const, override));
+    MOCK_METHOD(void, dumpOverlayProperties, (std::string&), (const, override));
+    MOCK_METHOD(android::Hwc2::Composer*, getComposer, (), (const, override));
+
+    MOCK_METHOD(hal::HWDisplayId, getPrimaryHwcDisplayId, (), (const, override));
+    MOCK_METHOD(PhysicalDisplayId, getPrimaryDisplayId, (), (const, override));
+    MOCK_METHOD(bool, isHeadless, (), (const, override));
+
+    MOCK_METHOD(std::optional<PhysicalDisplayId>, toPhysicalDisplayId, (hal::HWDisplayId),
+                (const, override));
+    MOCK_METHOD(std::optional<hal::HWDisplayId>, fromPhysicalDisplayId, (PhysicalDisplayId),
+                (const, override));
+    MOCK_METHOD(status_t, getDisplayDecorationSupport,
+                (PhysicalDisplayId,
+                 std::optional<aidl::android::hardware::graphics::common::DisplayDecorationSupport>*
+                         support),
+                (override));
+    MOCK_METHOD(status_t, setIdleTimerEnabled, (PhysicalDisplayId, std::chrono::milliseconds),
+                (override));
+    MOCK_METHOD(bool, hasDisplayIdleTimerCapability, (PhysicalDisplayId), (const, override));
+    MOCK_METHOD(Hwc2::AidlTransform, getPhysicalDisplayOrientation, (PhysicalDisplayId),
+                (const, override));
+    MOCK_METHOD(bool, getValidateSkipped, (HalDisplayId), (const, override));
+    MOCK_METHOD(const aidl::android::hardware::graphics::composer3::OverlayProperties&,
+                getOverlaySupport, (), (const, override));
+    MOCK_METHOD(status_t, setRefreshRateChangedCallbackDebugEnabled, (PhysicalDisplayId, bool));
+    MOCK_METHOD(status_t, notifyExpectedPresent, (PhysicalDisplayId, TimePoint, Fps));
+    MOCK_METHOD(HWC2::Display::LutFileDescriptorMapper&, getLutFileDescriptorMapper, (),
+                (override));
+};
+
+} // namespace android::mock