No longer use java.applet as it's scheduled for removal.

minecraft/src/main/java/net/fabricmc/loader/impl/game/minecraft/MinecraftGameProvider.java

@@ -77,6 +77,7 @@ public class MinecraftGameProvider implements GameProvider {
 
 	private EnvType envType;
 	private String entrypoint;
+	private boolean usesApplet;
 	private Arguments arguments;
 	private final List<Path> gameJars = new ArrayList<>(2); // env game jar and potentially common game jar
 	private Path realmsJar;
@@ -87,6 +88,8 @@ public class MinecraftGameProvider implements GameProvider {
 	private Collection<Path> validParentClassPath; // computed parent class path restriction (loader+deps)
 	private McVersion versionData;
 	private boolean hasModLoader = false;
+	private Set<BuiltinTransform> gameTransforms;
+	private Set<BuiltinTransform> modTransforms;
 
 	private final GameTransformer transformer = new GameTransformer(
 			new EntrypointPatch(this),
@@ -164,19 +167,12 @@ public Set<BuiltinTransform> getBuiltinTransforms(String className) {
 				|| className.indexOf('.') < 0; // obf classes
 
 		if (isMinecraftClass) {
-			if (FabricLoaderImpl.INSTANCE.isDevelopmentEnvironment()) { // combined client+server jar, strip back down to production equivalent
-				return TRANSFORM_WIDENALL_STRIPENV_CLASSTWEAKS;
-			} else { // environment specific jar, inherently env stripped
-				return TRANSFORM_WIDENALL_CLASSTWEAKS;
-			}
-		} else { // mod class TODO: exclude game libs
-			return TRANSFORM_STRIPENV;
+			return gameTransforms;
 		}
-	}
 
-	private static final Set<BuiltinTransform> TRANSFORM_WIDENALL_STRIPENV_CLASSTWEAKS = EnumSet.of(BuiltinTransform.WIDEN_ALL_PACKAGE_ACCESS, BuiltinTransform.STRIP_ENVIRONMENT, BuiltinTransform.CLASS_TWEAKS);
-	private static final Set<BuiltinTransform> TRANSFORM_WIDENALL_CLASSTWEAKS = EnumSet.of(BuiltinTransform.WIDEN_ALL_PACKAGE_ACCESS, BuiltinTransform.CLASS_TWEAKS);
-	private static final Set<BuiltinTransform> TRANSFORM_STRIPENV = EnumSet.of(BuiltinTransform.STRIP_ENVIRONMENT);
+		// mod class TODO: exclude game libs
+		return modTransforms;
+	}
 
 	@Override
 	public boolean isEnabled() {
@@ -234,10 +230,14 @@ public boolean locateGame(FabricLauncher launcher, String[] args) {
 			}
 
 			entrypoint = classifier.getClassName(envGameLib);
+			usesApplet = envType == EnvType.CLIENT && entrypoint != null && entrypoint.contains("Applet");
 			realmsJar = classifier.getOrigin(McLibrary.REALMS);
 			hasModLoader = classifier.has(McLibrary.MODLOADER);
 			log4jAvailable = classifier.has(McLibrary.LOG4J_API) && classifier.has(McLibrary.LOG4J_CORE);
 			slf4jAvailable = classifier.has(McLibrary.SLF4J_API) && classifier.has(McLibrary.SLF4J_CORE);
+			gameTransforms = getGameTransforms(FabricLoaderImpl.INSTANCE.isDevelopmentEnvironment(), usesApplet);
+			modTransforms = EnumSet.of(BuiltinTransform.STRIP_ENVIRONMENT);
+
 			boolean hasLogLib = log4jAvailable || slf4jAvailable;
 
 			Log.configureBuiltin(hasLogLib, !hasLogLib);
@@ -489,7 +489,7 @@ public void unlockClassPath(FabricLauncher launcher) {
 	public void launch(ClassLoader loader) {
 		String targetClass = entrypoint;
 
-		if (envType == EnvType.CLIENT && targetClass.contains("Applet")) {
+		if (usesApplet) {
 			targetClass = "net.fabricmc.loader.impl.game.minecraft.applet.AppletMain";
 		}
 
@@ -508,4 +508,18 @@ public void launch(ClassLoader loader) {
 			throw FormattedException.ofLocalized("exception.minecraft.generic", t);
 		}
 	}
+
+	private static Set<BuiltinTransform> getGameTransforms(boolean devEnv, boolean applet) {
+		EnumSet<BuiltinTransform> transforms = EnumSet.of(BuiltinTransform.WIDEN_ALL_PACKAGE_ACCESS, BuiltinTransform.CLASS_TWEAKS);
+
+		if (devEnv) {
+			transforms.add(BuiltinTransform.STRIP_ENVIRONMENT);
+		}
+
+		if (applet) {
+			transforms.add(BuiltinTransform.MINECRAFT_APPLET_STUB);
+		}
+
+		return Collections.unmodifiableSet(transforms);
+	}
 }

minecraft/src/main/java/net/fabricmc/loader/impl/game/minecraft/applet/AppletLauncher.java

@@ -16,8 +16,6 @@
 
 package net.fabricmc.loader.impl.game.minecraft.applet;
 
-import java.applet.Applet;
-import java.applet.AppletStub;
 import java.awt.BorderLayout;
 import java.awt.Dimension;
 import java.io.File;
@@ -28,6 +26,8 @@
 import java.util.Map;
 
 import net.fabricmc.loader.impl.game.minecraft.Hooks;
+import net.fabricmc.loader.impl.game.minecraft.applet.stub.Applet;
+import net.fabricmc.loader.impl.game.minecraft.applet.stub.AppletStub;
 import net.fabricmc.loader.impl.launch.FabricLauncherBase;
 
 /**
@@ -38,7 +38,6 @@
  *
  * <p>It has been adapted here for the purposes of the Fabric loader.
  */
-@SuppressWarnings("serial")
 public class AppletLauncher extends Applet implements AppletStub {
 	public static File gameDir;
 
@@ -95,11 +94,6 @@ public void replace(Applet applet) {
 		}
 	}
 
-	@Override
-	public void appletResize(int width, int height) {
-		mcApplet.resize(width, height);
-	}
-
 	@Override
 	public void resize(int width, int height) {
 		mcApplet.resize(width, height);

minecraft/src/main/java/net/fabricmc/loader/impl/game/minecraft/applet/AppletMain.java

@@ -35,6 +35,9 @@ public static File hookGameDir(File file) {
 	}
 
 	public static void main(String[] args) {
+		// Fix for border when using a HiDPI display
+		System.setProperty("sun.java2d.uiScale", "1.0");
+
 		java.awt.EventQueue.invokeLater(new AppletMain(args));
 	}
 

minecraft/src/main/java/net/fabricmc/loader/impl/game/minecraft/applet/stub/Applet.java

@@ -0,0 +1,101 @@
+/*
+ * Copyright 2016 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.loader.impl.game.minecraft.applet.stub;
+
+import java.awt.Dimension;
+import java.awt.Panel;
+import java.net.URL;
+import java.util.Locale;
+
+import org.jetbrains.annotations.Nullable;
+
+import net.fabricmc.loader.impl.game.minecraft.applet.AppletLauncher;
+
+/**
+ * A minimal stub implementation of an Applet, just enough to make Minecraft happy.
+ */
+public class Applet extends Panel {
+	private @Nullable AppletLauncher appletLauncher;
+
+	public final void setStub(AppletLauncher appletLauncher) {
+		this.appletLauncher = appletLauncher;
+	}
+
+	public boolean isActive() {
+		if (appletLauncher != null) {
+			return appletLauncher.isActive();
+		} else {
+			return false;
+		}
+	}
+
+	public URL getDocumentBase() {
+		return appletLauncher.getDocumentBase();
+	}
+
+	public URL getCodeBase() {
+		return appletLauncher.getCodeBase();
+	}
+
+	public String getParameter(String name) {
+		return appletLauncher.getParameter(name);
+	}
+
+	@SuppressWarnings("deprecation")
+	@Override
+	public void resize(int width, int height) {
+		Dimension d = size();
+
+		if ((d.width != width) || (d.height != height)) {
+			super.resize(width, height);
+		}
+	}
+
+	@SuppressWarnings("deprecation")
+	@Override
+	public void resize(Dimension d) {
+		resize(d.width, d.height);
+	}
+
+	@Override
+	public boolean isValidateRoot() {
+		return true;
+	}
+
+	@Override
+	public Locale getLocale() {
+		Locale locale = super.getLocale();
+
+		if (locale == null) {
+			return Locale.getDefault();
+		}
+
+		return locale;
+	}
+
+	public void init() {
+	}
+
+	public void start() {
+	}
+
+	public void stop() {
+	}
+
+	public void destroy() {
+	}
+}

minecraft/src/main/java/net/fabricmc/loader/impl/game/minecraft/applet/stub/AppletStub.java

@@ -0,0 +1,20 @@
+/*
+ * Copyright 2016 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.loader.impl.game.minecraft.applet.stub;
+
+public interface AppletStub {
+}

src/main/java/net/fabricmc/loader/impl/game/GameProvider.java

@@ -57,6 +57,10 @@ enum BuiltinTransform {
 		 * Applies class tweakers, including access wideners, as supplied by mods.
 		 */
 		CLASS_TWEAKS,
+		/**
+		 * Redirect Java applet usage to Fabric Loader's own stub classes, used to support Java 26+ for Minecraft versions that use applets.
+		 */
+		MINECRAFT_APPLET_STUB,
 	}
 
 	boolean isEnabled();

src/main/java/net/fabricmc/loader/impl/transformer/FabricTransformer.java

@@ -34,8 +34,9 @@ public static byte[] transform(boolean isDevelopment, EnvType envType, String na
 		boolean transformAccess = transforms.contains(BuiltinTransform.WIDEN_ALL_PACKAGE_ACCESS) && FabricLauncherBase.getLauncher().getMappingConfiguration().requiresPackageAccessHack();
 		boolean environmentStrip = transforms.contains(BuiltinTransform.STRIP_ENVIRONMENT);
 		boolean applyAccessWidener = transforms.contains(BuiltinTransform.CLASS_TWEAKS) && FabricLoaderImpl.INSTANCE.getAccessWidener().getTargets().contains(name);
+		boolean appletStub = transforms.contains(BuiltinTransform.MINECRAFT_APPLET_STUB);
 
-		if (!transformAccess && !environmentStrip && !applyAccessWidener) {
+		if (!transformAccess && !environmentStrip && !applyAccessWidener && !appletStub) {
 			return bytes;
 		}
 
@@ -68,6 +69,11 @@ public static byte[] transform(boolean isDevelopment, EnvType envType, String na
 			}
 		}
 
+		if (appletStub) {
+			visitor = new MinecraftAppletTransformer(visitor);
+			visitorCount++;
+		}
+
 		if (visitorCount <= 0) {
 			return bytes;
 		}

src/main/java/net/fabricmc/loader/impl/transformer/MinecraftAppletTransformer.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2016 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.loader.impl.transformer;
+
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.Map;
+
+import org.objectweb.asm.ClassVisitor;
+import org.objectweb.asm.commons.ClassRemapper;
+import org.objectweb.asm.commons.Remapper;
+import org.objectweb.asm.commons.SimpleRemapper;
+
+import net.fabricmc.loader.impl.FabricLoaderImpl;
+
+public class MinecraftAppletTransformer extends ClassRemapper {
+	private static final Remapper REMAPPER = new SimpleRemapper(getRenames());
+
+	private static Map<String, String> getRenames() {
+		Map<String, String> renames = new HashMap<>();
+
+		// TODO move this transformer into the minecraft specific code.
+		renames.put("java/applet/Applet", "net/fabricmc/loader/impl/game/minecraft/applet/stub/Applet");
+		renames.put("java/applet/AppletStub", "net/fabricmc/loader/impl/game/minecraft/applet/stub/AppletStub");
+
+		return Collections.unmodifiableMap(renames);
+	}
+
+	public MinecraftAppletTransformer(ClassVisitor classVisitor) {
+		super(FabricLoaderImpl.ASM_VERSION, classVisitor, REMAPPER);
+	}
+}