Fix some logical bugs related to the Grindstone Event (#9089)

src/main/java/net/minecraftforge/common/ForgeHooks.java

@@ -759,7 +759,11 @@ public class ForgeHooks
     public static int onGrindstoneChange(@NotNull ItemStack top, @NotNull ItemStack bottom, Container outputSlot, int xp)
     {
         GrindstoneEvent.OnplaceItem e = new GrindstoneEvent.OnplaceItem(top, bottom, xp);
-        if (MinecraftForge.EVENT_BUS.post(e)) return e.getXp();
+        if (MinecraftForge.EVENT_BUS.post(e))
+        {
+            outputSlot.setItem(0, ItemStack.EMPTY);
+            return -1;
+        }
         if (e.getOutput().isEmpty()) return Integer.MIN_VALUE;
 
         outputSlot.setItem(0, e.getOutput());

src/main/java/net/minecraftforge/event/GrindstoneEvent.java

@@ -41,10 +41,7 @@ public abstract class GrindstoneEvent extends Event
     }
 
     /**
-     * This is the experience amount determined by the event, not the vanilla behavior. <br>
-     * If you are the first receiver of this event, it is guaranteed to be -1. <br>
-     * The value must be set to a positive value to override vanilla behavior. <br>
-     * if the value is equal to -1, the vanilla behavior for calculating experience will run. <br>
+     * This is the experience amount determined by the event. It will be {@code -1} unless {@link #setXp(int)} is called. <br>
      * @return The experience amount given to the player. <br>
      */
     public int getXp()
@@ -64,11 +61,22 @@ public abstract class GrindstoneEvent extends Event
     /**
      * This event is {@link Cancelable} <br>
      * {@link OnplaceItem} is fired when the inputs to a grindstone are changed. <br>
-     * It is called from {@link GrindstoneMenu#createResult()}. <br>
-     * If the event is canceled, vanilla behavior will not run, and the output will be set to {@link ItemStack#EMPTY}. <br>
-     * If the event is not canceled, but the output is not empty, it will set the output and not run vanilla behavior. <br>
-     * if the output is empty, and the event is not canceled, vanilla behavior will execute. <br>
-     * if the amount of experience is larger than or equal 0, the vanilla behavior for calculating experience will not run. <br>
+     *
+     * The following rules apply:
+     * <ul>
+     *     <li>If the event is canceled, vanilla behavior will not run, and the output will be {@linkplain ItemStack#EMPTY empty}.</li>
+     *     <li>If the event is not canceled</li>
+     *     <ul>
+     *          <li>and the output is empty, the output will be determined by vanilla.</li>
+     *          <li>and the output is not empty, the output will be set, without running vanilla behavior.</li>
+     *     </ul>
+     *     <li>Vanilla XP calculation logic will be used unless all of the following criterias are met:</li>
+     *     <ul>
+     *         <li>the amount of experience is greater than or equal to {@code 0};</li>
+     *         <li>the event is not {@linkplain #isCanceled() canceled};</li>
+     *         <li>the {@linkplain #getOutput() output} is not empty.</li>
+     *     </ul>
+     * </ul>
      */
     @Cancelable
     public static class OnplaceItem extends GrindstoneEvent