Implement GUI Stacking feature. (#8130)

You can now use `mc.pushGuiLayer` and `mc.popGuiLayer` to manage the layers.
mc.setScreen behaves such that if called with a non-null screen it replaces the entire stack, and if called with null it closes the entire stack.

src/main/java/net/minecraftforge/client/ForgeHooksClient.java

@@ -23,6 +23,7 @@ import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.mojang.blaze3d.systems.RenderSystem;
 import com.mojang.blaze3d.vertex.*;
+import net.minecraft.client.gui.chat.NarratorChatListener;
 import net.minecraft.client.gui.screens.multiplayer.JoinMultiplayerScreen;
 import net.minecraft.client.model.geom.ModelLayerLocation;
 import net.minecraft.client.model.geom.builders.LayerDefinition;
@@ -127,7 +128,16 @@ import java.io.File;
 import java.io.IOException;
 import java.nio.Buffer;
 import java.nio.ByteBuffer;
-import java.util.*;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Random;
+import java.util.Set;
+import java.util.Stack;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
@@ -150,6 +160,55 @@ public class ForgeHooksClient
 
     //private static final ResourceLocation ITEM_GLINT = new ResourceLocation("textures/misc/enchanted_item_glint.png");
 
+    /**
+     * Contains the *extra* GUI layers.
+     * The current top layer stays in Minecraft#currentScreen, and the rest serve as a background for it.
+     */
+    private static final Stack<Screen> guiLayers = new Stack<>();
+
+    public static void clearGuiLayers(Minecraft minecraft)
+    {
+        while(guiLayers.size() > 0)
+            popGuiLayerInternal(minecraft);
+    }
+
+    private static void popGuiLayerInternal(Minecraft minecraft)
+    {
+        if (minecraft.screen != null)
+            minecraft.screen.removed();
+        minecraft.screen = guiLayers.pop();
+    }
+
+    public static void pushGuiLayer(Minecraft minecraft, Screen screen)
+    {
+        if (minecraft.screen != null)
+            guiLayers.push(minecraft.screen);
+        minecraft.screen = Objects.requireNonNull(screen);
+        screen.init(minecraft, minecraft.getWindow().getGuiScaledWidth(), minecraft.getWindow().getGuiScaledHeight());
+        NarratorChatListener.INSTANCE.sayNow(screen.getNarrationMessage());
+    }
+
+    public static void popGuiLayer(Minecraft minecraft)
+    {
+        if (guiLayers.size() == 0)
+        {
+            minecraft.setScreen(null);
+            return;
+        }
+
+        popGuiLayerInternal(minecraft);
+        if (minecraft.screen != null)
+            NarratorChatListener.INSTANCE.sayNow(minecraft.screen.getNarrationMessage());
+    }
+
+    public static float getGuiFarPlane()
+    {
+        // 1000 units for the overlay background,
+        // and 2000 units for each layered Screen,
+
+        return 1000.0F + 2000.0F * (1 + guiLayers.size());
+    }
+
     public static String getArmorTexture(Entity entity, ItemStack armor, String _default, EquipmentSlot slot, String type)
     {
         String result = armor.getItem().getArmorTexture(armor, entity, slot, type);
@@ -310,6 +369,18 @@ public class ForgeHooksClient
     }
 
     public static void drawScreen(Screen screen, PoseStack mStack, int mouseX, int mouseY, float partialTicks)
+    {
+        mStack.pushPose();
+        guiLayers.forEach(layer -> {
+            // Prevent the background layers from thinking the mouse is over their controls and showing them as highlighted.
+            drawScreenInternal(layer, mStack, Integer.MAX_VALUE, Integer.MAX_VALUE, partialTicks);
+            mStack.translate(0,0,2000);
+        });
+        drawScreenInternal(screen, mStack, mouseX, mouseY, partialTicks);
+        mStack.popPose();
+    }
+
+    private static void drawScreenInternal(Screen screen, PoseStack mStack, int mouseX, int mouseY, float partialTicks)
     {
         if (!MinecraftForge.EVENT_BUS.post(new GuiScreenEvent.DrawScreenEvent.Pre(screen, mStack, mouseX, mouseY, partialTicks)))
             screen.render(mStack, mouseX, mouseY, partialTicks);

src/main/java/net/minecraftforge/client/extensions/IForgeMinecraft.java

@@ -0,0 +1,39 @@
+/*
+ * Minecraft Forge
+ * Copyright (c) 2016-2021.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation version 2.1
+ * of the License.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+
+package net.minecraftforge.client.extensions;
+
+import net.minecraft.client.Minecraft;
+import net.minecraft.client.gui.screens.Screen;
+import net.minecraftforge.client.ForgeHooksClient;
+
+public interface IForgeMinecraft
+{
+    private Minecraft self() { return (Minecraft)this; }
+
+    default void pushGuiLayer(Screen screen)
+    {
+        ForgeHooksClient.pushGuiLayer(self(), screen);
+    }
+
+    default void popGuiLayer()
+    {
+        ForgeHooksClient.popGuiLayer(self());
+    }
+}