Client gametest screenshot changes (#4329)

* Instant screenshots, with counter and more configurability

* Force consistent window size across all systems and stretch framebuffer to fit the physical window

* Docs

* Wait ticks appropriately for the screenshots in the gametest test

* Should -> must

* Fix window resizing for different display scales

* Fix framebuffer size not being changed at all

* Add test for window resizing

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/api/client/gametest/v1/ClientGameTestContext.java

@@ -111,19 +111,20 @@ public interface ClientGameTestContext {
 	boolean tryClickScreenButton(String translationKey);
 
 	/**
-	 * Takes a screenshot after waiting 1 tick (for a frame to render) and saves it in the screenshots directory.
+	 * Takes a screenshot and saves it in the screenshots directory.
 	 *
 	 * @param name The name of the screenshot
+	 * @return The {@link Path} to the screenshot
 	 */
 	Path takeScreenshot(String name);
 
 	/**
-	 * Takes a screnshot after waiting {@code delay} ticks and saves it in the screenshots directory.
+	 * Takes a screenshot with the given options.
 	 *
-	 * @param name The name of the screenshot
-	 * @param delay The delay in ticks before taking the screenshot
+	 * @param options The {@link TestScreenshotOptions} to take the screenshot with
+	 * @return The {@link Path} to the screenshot
 	 */
-	Path takeScreenshot(String name, int delay);
+	Path takeScreenshot(TestScreenshotOptions options);
 
 	/**
 	 * Gets the input handler used to simulate inputs to the client.

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/api/client/gametest/v1/TestInput.java

@@ -325,4 +325,14 @@ public interface TestInput {
 	 * @see #setCursorPos(double, double)
 	 */
 	void moveCursor(double deltaX, double deltaY);
+
+	/**
+	 * Resizes the window to match the given size. Also attempts to resize the physical window, but whether the physical
+	 * window was successfully resized or not, the window size accessible by the game will always be changed to the
+	 * value specified, causing widget layouts and screenshots to work as expected.
+	 *
+	 * @param width The new window width
+	 * @param height The new window height
+	 */
+	void resizeWindow(int width, int height);
 }

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/api/client/gametest/v1/TestScreenshotOptions.java

@@ -0,0 +1,77 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * 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.
+ */
+
+package net.fabricmc.fabric.api.client.gametest.v1;
+
+import java.nio.file.Path;
+
+import com.google.common.base.Preconditions;
+import org.jetbrains.annotations.ApiStatus;
+
+import net.fabricmc.fabric.impl.client.gametest.TestScreenshotOptionsImpl;
+
+/**
+ * Options to customize a screenshot.
+ */
+@ApiStatus.NonExtendable
+public interface TestScreenshotOptions {
+	/**
+	 * Creates a {@link TestScreenshotOptions} with the given screenshot name.
+	 *
+	 * @param name The name of the screenshot
+	 * @return The new screenshot options instance
+	 */
+	static TestScreenshotOptions of(String name) {
+		Preconditions.checkNotNull(name, "name");
+		return new TestScreenshotOptionsImpl(name);
+	}
+
+	/**
+	 * By default, screenshot file names will be prefixed by a counter so that the screenshots appear in sequence in the
+	 * screenshots directory. Use this method to disable this behavior.
+	 *
+	 * @return This screenshot options instance
+	 */
+	TestScreenshotOptions disableCounterPrefix();
+
+	/**
+	 * Changes the tick delta to take this screenshot with. Tick delta controls interpolation between the previous tick and the
+	 * current tick to make objects appear to move more smoothly when there are multiple frames in a tick. Defaults to
+	 * {@code 1}, which renders all objects as their appear in the current tick.
+	 *
+	 * @param tickDelta The tick delta to take this screenshot with
+	 * @return This screenshot options instance
+	 */
+	TestScreenshotOptions withTickDelta(float tickDelta);
+
+	/**
+	 * Changes the resolution of the screenshot, which defaults to the resolution of the Minecraft window.
+	 *
+	 * @param width The width of the screenshot
+	 * @param height The height of the screenshot
+	 * @return This screenshot options instance
+	 */
+	TestScreenshotOptions withSize(int width, int height);
+
+	/**
+	 * Changes the directory in which this screenshot is saved, which defaults to the {@code screenshots} directory in
+	 * the game's run directory.
+	 *
+	 * @param destinationDir The directory in which to save the screenshot
+	 * @return This screenshot options instance
+	 */
+	TestScreenshotOptions withDestinationDir(Path destinationDir);
+}

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/impl/client/gametest/ClientGameTestContextImpl.java

@@ -16,9 +16,12 @@
 
 package net.fabricmc.fabric.impl.client.gametest;
 
+import java.io.IOException;
+import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Objects;
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.function.Supplier;
@@ -41,6 +44,7 @@ import net.minecraft.client.gui.widget.Widget;
 import net.minecraft.client.option.CloudRenderMode;
 import net.minecraft.client.option.GameOptions;
 import net.minecraft.client.option.SimpleOption;
+import net.minecraft.client.texture.NativeImage;
 import net.minecraft.client.tutorial.TutorialStep;
 import net.minecraft.client.util.ScreenshotRecorder;
 import net.minecraft.sound.SoundCategory;
@@ -48,9 +52,11 @@ import net.minecraft.text.Text;
 import net.minecraft.util.Nullables;
 
 import net.fabricmc.fabric.api.client.gametest.v1.ClientGameTestContext;
+import net.fabricmc.fabric.api.client.gametest.v1.TestScreenshotOptions;
 import net.fabricmc.fabric.api.client.gametest.v1.TestWorldBuilder;
 import net.fabricmc.fabric.mixin.client.gametest.CyclingButtonWidgetAccessor;
 import net.fabricmc.fabric.mixin.client.gametest.GameOptionsAccessor;
+import net.fabricmc.fabric.mixin.client.gametest.RenderTickCounterConstantAccessor;
 import net.fabricmc.fabric.mixin.client.gametest.ScreenAccessor;
 import net.fabricmc.loader.api.FabricLoader;
 
@@ -262,22 +268,56 @@ public final class ClientGameTestContextImpl implements ClientGameTestContext {
 	public Path takeScreenshot(String name) {
 		ThreadingImpl.checkOnGametestThread("takeScreenshot");
 		Preconditions.checkNotNull(name, "name");
-		return takeScreenshot(name, 1);
+		return takeScreenshot(TestScreenshotOptions.of(name));
 	}
 
 	@Override
-	public Path takeScreenshot(String name, int delay) {
+	public Path takeScreenshot(TestScreenshotOptions options) {
 		ThreadingImpl.checkOnGametestThread("takeScreenshot");
-		Preconditions.checkNotNull(name, "name");
-		Preconditions.checkArgument(delay >= 0, "delay cannot be negative");
+		Preconditions.checkNotNull(options, "options");
 
-		waitTicks(delay);
-		runOnClient(client -> {
-			ScreenshotRecorder.saveScreenshot(FabricLoader.getInstance().getGameDir().toFile(), name + ".png", client.getFramebuffer(), (message) -> {
-			});
+		TestScreenshotOptionsImpl optionsImpl = (TestScreenshotOptionsImpl) options;
+		return computeOnClient(client -> {
+			int prevWidth = client.getWindow().getFramebufferWidth();
+			int prevHeight = client.getWindow().getFramebufferHeight();
+
+			if (optionsImpl.size != null) {
+				client.getWindow().setFramebufferWidth(optionsImpl.size.x);
+				client.getWindow().setFramebufferHeight(optionsImpl.size.y);
+				client.getFramebuffer().resize(optionsImpl.size.x, optionsImpl.size.y);
+			}
+
+			try {
+				client.gameRenderer.render(RenderTickCounterConstantAccessor.create(optionsImpl.tickDelta), true);
+
+				// The vanilla panorama screenshot code has a Thread.sleep(10) here, is this needed?
+
+				Path destinationDir = Objects.requireNonNullElseGet(optionsImpl.destinationDir, () -> FabricLoader.getInstance().getGameDir().resolve("screenshots"));
+
+				try {
+					Files.createDirectories(destinationDir);
+				} catch (IOException e) {
+					throw new AssertionError("Failed to create screenshots directory", e);
+				}
+
+				String counterPrefix = optionsImpl.counterPrefix ? "%04d_".formatted(ClientGameTestImpl.screenshotCounter++) : "";
+				Path screenshotFile = destinationDir.resolve(counterPrefix + optionsImpl.name + ".png");
+
+				try (NativeImage screenshot = ScreenshotRecorder.takeScreenshot(client.getFramebuffer())) {
+					screenshot.writeTo(screenshotFile);
+				} catch (IOException e) {
+					throw new AssertionError("Failed to write screenshot file", e);
+				}
+
+				return screenshotFile;
+			} finally {
+				if (optionsImpl.size != null) {
+					client.getWindow().setFramebufferWidth(prevWidth);
+					client.getWindow().setFramebufferHeight(prevHeight);
+					client.getFramebuffer().resize(prevWidth, prevHeight);
+				}
+			}
 		});
-
-		return FabricLoader.getInstance().getGameDir().resolve("screenshots").resolve(name + ".png");
 	}
 
 	@Override

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/impl/client/gametest/ClientGameTestImpl.java

@@ -31,6 +31,7 @@ import net.fabricmc.fabric.api.client.gametest.v1.ClientGameTestContext;
 
 public final class ClientGameTestImpl {
 	public static final Logger LOGGER = LoggerFactory.getLogger("fabric-client-gametest-api-v1");
+	public static int screenshotCounter = 0;
 
 	private ClientGameTestImpl() {
 	}

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/impl/client/gametest/FabricClientGameTestRunner.java

@@ -38,17 +38,24 @@ public class FabricClientGameTestRunner {
 			ClientGameTestContextImpl context = new ClientGameTestContextImpl();
 
 			for (FabricClientGameTest gameTest : gameTests) {
-				context.restoreDefaultGameOptions();
+				setupInitialGameTestState(context);
 
 				gameTest.runTest(context);
 
-				context.getInput().clearKeysDown();
-				checkFinalGameTestState(context, gameTest.getClass().getName());
+				setupAndCheckFinalGameTestState(context, gameTest.getClass().getName());
 			}
 		});
 	}
 
-	private static void checkFinalGameTestState(ClientGameTestContext context, String testClassName) {
+	private static void setupInitialGameTestState(ClientGameTestContext context) {
+		context.restoreDefaultGameOptions();
+	}
+
+	private static void setupAndCheckFinalGameTestState(ClientGameTestContextImpl context, String testClassName) {
+		context.getInput().clearKeysDown();
+		context.runOnClient(client -> ((WindowHooks) (Object) client.getWindow()).fabric_resetSize());
+		context.getInput().setCursorPos(context.computeOnClient(client -> client.getWindow().getWidth()) * 0.5, context.computeOnClient(client -> client.getWindow().getHeight()) * 0.5);
+
 		if (ThreadingImpl.isServerRunning) {
 			throw new AssertionError("Client gametest %s finished while a server is still running".formatted(testClassName));
 		}

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/impl/client/gametest/TestInputImpl.java

@@ -321,6 +321,15 @@ public final class TestInputImpl implements TestInput {
 		});
 	}
 
+	@Override
+	public void resizeWindow(int width, int height) {
+		ThreadingImpl.checkOnGametestThread("resizeWindow");
+		Preconditions.checkArgument(width > 0, "width must be positive");
+		Preconditions.checkArgument(height > 0, "height must be positive");
+
+		context.runOnClient(client -> ((WindowHooks) (Object) client.getWindow()).fabric_resize(width, height));
+	}
+
 	private static InputUtil.Key getBoundKey(KeyBinding keyBinding, String action) {
 		InputUtil.Key boundKey = ((KeyBindingAccessor) keyBinding).getBoundKey();
 

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/impl/client/gametest/TestScreenshotOptionsImpl.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * 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.
+ */
+
+package net.fabricmc.fabric.impl.client.gametest;
+
+import java.nio.file.Path;
+
+import com.google.common.base.Preconditions;
+import org.jetbrains.annotations.Nullable;
+import org.joml.Vector2i;
+
+import net.fabricmc.fabric.api.client.gametest.v1.TestScreenshotOptions;
+
+public class TestScreenshotOptionsImpl implements TestScreenshotOptions {
+	public final String name;
+	public boolean counterPrefix = true;
+	public float tickDelta = 1;
+	@Nullable
+	public Vector2i size;
+	@Nullable
+	public Path destinationDir;
+
+	public TestScreenshotOptionsImpl(String name) {
+		this.name = name;
+	}
+
+	@Override
+	public TestScreenshotOptions disableCounterPrefix() {
+		counterPrefix = false;
+		return this;
+	}
+
+	@Override
+	public TestScreenshotOptions withTickDelta(float tickDelta) {
+		Preconditions.checkArgument(tickDelta >= 0 && tickDelta <= 1, "tickDelta must be between 0 and 1");
+
+		this.tickDelta = tickDelta;
+		return this;
+	}
+
+	@Override
+	public TestScreenshotOptions withSize(int width, int height) {
+		Preconditions.checkArgument(width > 0, "width must be positive");
+		Preconditions.checkArgument(height > 0, "height must be positive");
+
+		this.size = new Vector2i(width, height);
+		return this;
+	}
+
+	@Override
+	public TestScreenshotOptions withDestinationDir(Path destinationDir) {
+		Preconditions.checkNotNull(destinationDir, "destinationDir");
+
+		this.destinationDir = destinationDir;
+		return this;
+	}
+}

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/impl/client/gametest/WindowHooks.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * 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.
+ */
+
+package net.fabricmc.fabric.impl.client.gametest;
+
+public interface WindowHooks {
+	int fabric_getRealWidth();
+	int fabric_getRealHeight();
+	int fabric_getRealFramebufferWidth();
+	int fabric_getRealFramebufferHeight();
+	void fabric_resetSize();
+	void fabric_resize(int width, int height);
+}

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/mixin/client/gametest/FramebufferMixin.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * 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.
+ */
+
+package net.fabricmc.fabric.mixin.client.gametest;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.ModifyVariable;
+
+import net.minecraft.client.MinecraftClient;
+import net.minecraft.client.gl.Framebuffer;
+import net.minecraft.client.util.Window;
+
+import net.fabricmc.fabric.impl.client.gametest.WindowHooks;
+
+@Mixin(Framebuffer.class)
+public class FramebufferMixin {
+	@ModifyVariable(method = {"draw", "drawInternal"}, at = @At("HEAD"), ordinal = 0, argsOnly = true)
+	private int modifyWidth(int width) {
+		Window window = MinecraftClient.getInstance().getWindow();
+
+		if ((Object) this == MinecraftClient.getInstance().getFramebuffer() && width == window.getFramebufferWidth()) {
+			return ((WindowHooks) (Object) window).fabric_getRealFramebufferWidth();
+		}
+
+		return width;
+	}
+
+	@ModifyVariable(method = {"draw", "drawInternal"}, at = @At("HEAD"), ordinal = 1, argsOnly = true)
+	private int modifyHeight(int height) {
+		Window window = MinecraftClient.getInstance().getWindow();
+
+		if ((Object) this == MinecraftClient.getInstance().getFramebuffer() && height == window.getFramebufferHeight()) {
+			return ((WindowHooks) (Object) window).fabric_getRealFramebufferHeight();
+		}
+
+		return height;
+	}
+}

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/mixin/client/gametest/MinecraftClientMixin.java

@@ -17,9 +17,11 @@
 package net.fabricmc.fabric.mixin.client.gametest;
 
 import com.google.common.base.Preconditions;
+import com.llamalad7.mixinextras.injector.ModifyExpressionValue;
 import com.llamalad7.mixinextras.injector.wrapmethod.WrapMethod;
 import com.llamalad7.mixinextras.injector.wrapoperation.Operation;
 import org.jetbrains.annotations.Nullable;
+import org.spongepowered.asm.mixin.Final;
 import org.spongepowered.asm.mixin.Mixin;
 import org.spongepowered.asm.mixin.Shadow;
 import org.spongepowered.asm.mixin.Unique;
@@ -31,12 +33,14 @@ import org.spongepowered.asm.mixin.injection.callback.CallbackInfoReturnable;
 import net.minecraft.client.MinecraftClient;
 import net.minecraft.client.gui.screen.Overlay;
 import net.minecraft.client.gui.screen.Screen;
+import net.minecraft.client.util.Window;
 import net.minecraft.resource.ResourcePackManager;
 import net.minecraft.server.SaveLoader;
 import net.minecraft.world.level.storage.LevelStorage;
 
 import net.fabricmc.fabric.impl.client.gametest.FabricClientGameTestRunner;
 import net.fabricmc.fabric.impl.client.gametest.ThreadingImpl;
+import net.fabricmc.fabric.impl.client.gametest.WindowHooks;
 
 @Mixin(MinecraftClient.class)
 public class MinecraftClientMixin {
@@ -49,6 +53,10 @@ public class MinecraftClientMixin {
 	@Nullable
 	private Overlay overlay;
 
+	@Shadow
+	@Final
+	private Window window;
+
 	@WrapMethod(method = "run")
 	private void onRun(Operation<Void> original) throws Throwable {
 		if (ThreadingImpl.isClientRunning) {
@@ -162,6 +170,12 @@ public class MinecraftClientMixin {
 		);
 	}
 
+	@ModifyExpressionValue(method = "render", at = @At(value = "INVOKE", target = "Lnet/minecraft/client/util/Window;hasZeroWidthOrHeight()Z"))
+	private boolean hasZeroRealWidthOrHeight(boolean original) {
+		WindowHooks windowHooks = (WindowHooks) (Object) window;
+		return windowHooks.fabric_getRealFramebufferWidth() == 0 || windowHooks.fabric_getRealFramebufferHeight() == 0;
+	}
+
 	@Unique
 	private static void deregisterClient() {
 		if (ThreadingImpl.isClientRunning) {

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/mixin/client/gametest/MonitorTrackerMixin.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * 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.
+ */
+
+package net.fabricmc.fabric.mixin.client.gametest;
+
+import com.llamalad7.mixinextras.injector.ModifyExpressionValue;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.injection.At;
+
+import net.minecraft.client.util.MonitorTracker;
+import net.minecraft.client.util.Window;
+
+import net.fabricmc.fabric.impl.client.gametest.WindowHooks;
+
+@Mixin(MonitorTracker.class)
+public class MonitorTrackerMixin {
+	@ModifyExpressionValue(method = "getMonitor(Lnet/minecraft/client/util/Window;)Lnet/minecraft/client/util/Monitor;", at = @At(value = "INVOKE", target = "Lnet/minecraft/client/util/Window;getWidth()I"))
+	private int getRealWidth(int original, Window window) {
+		return ((WindowHooks) (Object) window).fabric_getRealWidth();
+	}
+
+	@ModifyExpressionValue(method = "getMonitor(Lnet/minecraft/client/util/Window;)Lnet/minecraft/client/util/Monitor;", at = @At(value = "INVOKE", target = "Lnet/minecraft/client/util/Window;getHeight()I"))
+	private int getRealHeight(int original, Window window) {
+		return ((WindowHooks) (Object) window).fabric_getRealHeight();
+	}
+}

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/mixin/client/gametest/RenderTickCounterConstantAccessor.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2016, 2017, 2018, 2019 FabricMC
+ *
+ * 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.
+ */
+
+package net.fabricmc.fabric.mixin.client.gametest;
+
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.gen.Invoker;
+
+import net.minecraft.client.render.RenderTickCounter;
+
+@Mixin(RenderTickCounter.Constant.class)
+public interface RenderTickCounterConstantAccessor {
+	@Invoker("<init>")
+	static RenderTickCounter.Constant create(float constant) {
+		throw new UnsupportedOperationException("Implemented via mixin");
+	}
+}

fabric-client-gametest-api-v1/src/client/java/net/fabricmc/fabric/mixin/client/gametest/WindowMixin.java

@@ -16,17 +16,199 @@
 
 package net.fabricmc.fabric.mixin.client.gametest;
 
+import java.util.Optional;
+
+import com.llamalad7.mixinextras.injector.wrapmethod.WrapMethod;
+import com.llamalad7.mixinextras.injector.wrapoperation.Operation;
+import org.jetbrains.annotations.Nullable;
+import org.spongepowered.asm.mixin.Final;
 import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Shadow;
+import org.spongepowered.asm.mixin.Unique;
 import org.spongepowered.asm.mixin.injection.At;
 import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;
 
+import net.minecraft.client.WindowEventHandler;
+import net.minecraft.client.WindowSettings;
+import net.minecraft.client.util.Monitor;
+import net.minecraft.client.util.MonitorTracker;
+import net.minecraft.client.util.VideoMode;
 import net.minecraft.client.util.Window;
 
+import net.fabricmc.fabric.impl.client.gametest.WindowHooks;
+
 @Mixin(Window.class)
-public class WindowMixin {
-	@Inject(method = {"onWindowFocusChanged", "onCursorEnterChanged"}, at = @At("HEAD"), cancellable = true)
+public abstract class WindowMixin implements WindowHooks {
+	@Shadow
+	private int x;
+	@Shadow
+	private int y;
+	@Shadow
+	private int windowedX;
+	@Shadow
+	private int windowedY;
+	@Shadow
+	private int width;
+	@Shadow
+	private int height;
+	@Shadow
+	private int windowedWidth;
+	@Shadow
+	private int windowedHeight;
+	@Shadow
+	private int framebufferWidth;
+	@Shadow
+	private int framebufferHeight;
+	@Shadow
+	private boolean fullscreen;
+	@Shadow
+	@Final
+	private WindowEventHandler eventHandler;
+	@Shadow
+	@Final
+	private MonitorTracker monitorTracker;
+	@Shadow
+	private Optional<VideoMode> fullscreenVideoMode;
+
+	@Shadow
+	protected abstract void updateWindowRegion();
+
+	@Unique
+	private int defaultWidth;
+	@Unique
+	private int defaultHeight;
+	@Unique
+	private int realWidth;
+	@Unique
+	private int realHeight;
+	@Unique
+	private int realFramebufferWidth;
+	@Unique
+	private int realFramebufferHeight;
+
+	@Inject(method = "<init>", at = @At("RETURN"))
+	private void onInit(WindowEventHandler eventHandler, MonitorTracker monitorTracker, WindowSettings settings, @Nullable String fullscreenVideoMode, String title, CallbackInfo ci) {
+		this.defaultWidth = settings.width;
+		this.defaultHeight = settings.height;
+		this.realWidth = this.width;
+		this.realHeight = this.height;
+		this.realFramebufferWidth = this.framebufferWidth;
+		this.realFramebufferHeight = this.framebufferHeight;
+
+		this.width = this.windowedWidth = this.framebufferWidth = defaultWidth;
+		this.height = this.windowedHeight = this.framebufferHeight = defaultHeight;
+	}
+
+	@Inject(method = {"onWindowFocusChanged", "onCursorEnterChanged", "onMinimizeChanged"}, at = @At("HEAD"), cancellable = true)
 	private void cancelEvents(CallbackInfo ci) {
 		ci.cancel();
 	}
+
+	@Inject(method = "onWindowSizeChanged", at = @At("HEAD"), cancellable = true)
+	private void cancelWindowSizeChanged(long window, int width, int height, CallbackInfo ci) {
+		realWidth = width;
+		realHeight = height;
+		ci.cancel();
+	}
+
+	@Inject(method = "onFramebufferSizeChanged", at = @At("HEAD"), cancellable = true)
+	private void cancelFramebufferSizeChanged(long window, int width, int height, CallbackInfo ci) {
+		realFramebufferWidth = width;
+		realFramebufferHeight = height;
+		ci.cancel();
+	}
+
+	@WrapMethod(method = "updateWindowRegion")
+	private void wrapUpdateWindowRegion(Operation<Void> original) {
+		int prevWidth = this.width;
+		int prevHeight = this.height;
+		int prevWindowedWidth = this.windowedWidth;
+		int prevWindowedHeight = this.windowedHeight;
+
+		original.call();
+
+		this.realWidth = this.width;
+		this.realHeight = this.height;
+
+		this.width = prevWidth;
+		this.height = prevHeight;
+		this.windowedWidth = prevWindowedWidth;
+		this.windowedHeight = prevWindowedHeight;
+	}
+
+	@Inject(method = "setWindowedSize", at = @At("HEAD"), cancellable = true)
+	private void setWindowedSize(int width, int height, CallbackInfo ci) {
+		this.fullscreen = false;
+		fabric_resize(width, height);
+		ci.cancel();
+	}
+
+	@Override
+	public int fabric_getRealWidth() {
+		return realWidth;
+	}
+
+	@Override
+	public int fabric_getRealHeight() {
+		return realHeight;
+	}
+
+	@Override
+	public int fabric_getRealFramebufferWidth() {
+		return realFramebufferWidth;
+	}
+
+	@Override
+	public int fabric_getRealFramebufferHeight() {
+		return realFramebufferHeight;
+	}
+
+	@Override
+	public void fabric_resetSize() {
+		fabric_resize(defaultWidth, defaultHeight);
+	}
+
+	@Override
+	public void fabric_resize(int width, int height) {
+		if (width == this.width && width == this.windowedWidth && width == this.framebufferWidth && height == this.height && height == this.windowedHeight && height == this.framebufferHeight) {
+			return;
+		}
+
+		// Move the top left corner of the window so that the window expands/contracts from its center, while also
+		// trying to keep the window within the monitor's bounds
+		Monitor monitor = this.monitorTracker.getMonitor((Window) (Object) this);
+
+		if (monitor != null) {
+			VideoMode videoMode = monitor.findClosestVideoMode(this.fullscreenVideoMode);
+
+			this.x += (this.windowedWidth - width) / 2;
+			this.y += (this.windowedHeight - height) / 2;
+
+			if (this.x + width > monitor.getViewportX() + videoMode.getWidth()) {
+				this.x = monitor.getViewportX() + videoMode.getWidth() - width;
+			}
+
+			if (this.x < monitor.getViewportX()) {
+				this.x = monitor.getViewportX();
+			}
+
+			if (this.y + height > monitor.getViewportY() + videoMode.getHeight()) {
+				this.y = monitor.getViewportY() + videoMode.getHeight() - height;
+			}
+
+			if (this.y < monitor.getViewportY()) {
+				this.y = monitor.getViewportY();
+			}
+
+			this.windowedX = this.x;
+			this.windowedY = this.y;
+		}
+
+		this.width = this.windowedWidth = this.framebufferWidth = width;
+		this.height = this.windowedHeight = this.framebufferHeight = height;
+
+		updateWindowRegion();
+		this.eventHandler.onResolutionChanged();
+	}
 }

fabric-client-gametest-api-v1/src/client/resources/fabric-client-gametest-api-v1.mixins.json

@@ -26,6 +26,9 @@
     "ClientChunkMapAccessor",
     "ClientWorldAccessor",
     "CreateWorldScreenAccessor",
-    "CreateWorldScreenMixin"
+    "CreateWorldScreenMixin",
+    "FramebufferMixin",
+    "MonitorTrackerMixin",
+    "RenderTickCounterConstantAccessor"
   ]
 }

fabric-client-gametest-api-v1/src/testmodClient/java/net/fabricmc/fabric/test/client/gametest/ClientGameTestTest.java

@@ -16,13 +16,20 @@
 
 package net.fabricmc.fabric.test.client.gametest;
 
+import java.io.IOException;
+import java.io.UncheckedIOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
 import com.mojang.authlib.GameProfile;
 import org.spongepowered.asm.mixin.MixinEnvironment;
 
 import net.minecraft.client.MinecraftClient;
+import net.minecraft.client.gl.WindowFramebuffer;
 import net.minecraft.client.gui.screen.ReconfiguringScreen;
 import net.minecraft.client.gui.screen.world.WorldCreator;
 import net.minecraft.client.option.Perspective;
+import net.minecraft.client.texture.NativeImage;
 import net.minecraft.client.util.InputUtil;
 
 import net.fabricmc.fabric.api.client.gametest.v1.ClientGameTestContext;
@@ -37,7 +44,15 @@ public class ClientGameTestTest implements FabricClientGameTest {
 	public void runTest(ClientGameTestContext context) {
 		{
 			waitForTitleScreenFade(context);
-			context.takeScreenshot("title_screen", 0);
+			context.takeScreenshot("title_screen");
+		}
+
+		{
+			testScreenSize(context, WindowFramebuffer.DEFAULT_WIDTH, WindowFramebuffer.DEFAULT_HEIGHT);
+			context.getInput().resizeWindow(1000, 500);
+			context.waitTick();
+			testScreenSize(context, 1000, 500);
+			context.getInput().resizeWindow(WindowFramebuffer.DEFAULT_WIDTH, WindowFramebuffer.DEFAULT_HEIGHT);
 		}
 
 		TestWorldSave spWorldSave;
@@ -48,7 +63,7 @@ public class ClientGameTestTest implements FabricClientGameTest {
 			{
 				enableDebugHud(context);
 				singleplayer.getClientWorld().waitForChunksRender();
-				context.takeScreenshot("in_game_overworld", 0);
+				context.takeScreenshot("in_game_overworld");
 			}
 
 			{
@@ -56,7 +71,8 @@ public class ClientGameTestTest implements FabricClientGameTest {
 				context.waitTick();
 				context.getInput().typeChars("Hello, World!");
 				context.getInput().pressKey(InputUtil.GLFW_KEY_ENTER);
-				context.takeScreenshot("chat_message_sent", 5);
+				context.waitTick(); // wait for the server to receive the chat message
+				context.takeScreenshot("chat_message_sent");
 			}
 
 			MixinEnvironment.getCurrentEnvironment().audit();
@@ -70,6 +86,7 @@ public class ClientGameTestTest implements FabricClientGameTest {
 
 			{
 				context.getInput().pressKey(options -> options.inventoryKey);
+				context.waitTicks(2); // allow the client to process the key press, and then the server to receive the request
 				context.takeScreenshot("in_game_inventory");
 				context.setScreen(() -> null);
 			}
@@ -83,7 +100,7 @@ public class ClientGameTestTest implements FabricClientGameTest {
 		try (TestDedicatedServerContext server = context.worldBuilder().createServer()) {
 			try (TestServerConnection connection = server.connect()) {
 				connection.getClientWorld().waitForChunksRender();
-				context.takeScreenshot("server_in_game", 0);
+				context.takeScreenshot("server_in_game");
 
 				{ // Test that we can enter and exit configuration
 					final GameProfile profile = context.computeOnClient(MinecraftClient::getGameProfile);
@@ -111,4 +128,26 @@ public class ClientGameTestTest implements FabricClientGameTest {
 	private static void setPerspective(ClientGameTestContext context, Perspective perspective) {
 		context.runOnClient(client -> client.options.setPerspective(perspective));
 	}
+
+	private static void testScreenSize(ClientGameTestContext context, int expectedWidth, int expectedHeight) {
+		context.runOnClient(client -> {
+			if (client.getWindow().getWidth() != expectedWidth || client.getWindow().getHeight() != expectedHeight) {
+				throw new AssertionError("Expected window size to be (%d, %d) but was (%d, %d)".formatted(expectedWidth, expectedHeight, client.getWindow().getWidth(), client.getWindow().getHeight()));
+			}
+
+			if (client.getWindow().getFramebufferWidth() != expectedWidth || client.getWindow().getFramebufferHeight() != expectedHeight) {
+				throw new AssertionError("Expected framebuffer size to be (%d, %d) but was (%d, %d)".formatted(expectedWidth, expectedHeight, client.getWindow().getFramebufferWidth(), client.getWindow().getFramebufferHeight()));
+			}
+		});
+
+		Path screenshotPath = context.takeScreenshot("screenshot_size_test");
+
+		try (NativeImage screenshot = NativeImage.read(Files.newInputStream(screenshotPath))) {
+			if (screenshot.getWidth() != expectedWidth || screenshot.getHeight() != expectedHeight) {
+				throw new AssertionError("Expected screenshot size to be (%d, %d) but was (%d, %d)".formatted(expectedWidth, expectedHeight, screenshot.getWidth(), screenshot.getHeight()));
+			}
+		} catch (IOException e) {
+			throw new UncheckedIOException(e);
+		}
+	}
 }