Add handling for inserting items directly into trinket slots

src/main/java/com/minelittlepony/unicopia/mixin/trinkets/MixinScreenHandler.java

@@ -52,6 +52,16 @@ abstract class MixinScreenHandler {
         return sender.isEmpty() || (canApply(currentSlot) && (currentSlot.getStack().getCount() + sender.getCount()) <= currentSlot.getMaxItemCount(sender));
     }
 
+    @Redirect(method = "canInsertItemIntoSlot",
+            at = @At(
+                    value = "INVOKE",
+                    target = "Lnet/minecraft/item/ItemStack;getMaxCount()I"
+            )
+    )
+    private static int onCanInsertItemIntoSlot(ItemStack sender, @Nullable Slot slot) {
+        return canApply(slot) ? slot.getMaxItemCount(sender) : sender.getMaxCount();
+    }
+
     private static boolean canApply(Slot slot) {
         return slot instanceof TrinketSlot;
     }

src/main/java/com/minelittlepony/unicopia/mixin/trinkets/MixinTrinketInventory.java

@@ -0,0 +1,33 @@
+package com.minelittlepony.unicopia.mixin.trinkets;
+
+import org.spongepowered.asm.mixin.*;
+
+import com.minelittlepony.unicopia.trinkets.TrinketsDelegateImpl;
+import dev.emi.trinkets.api.*;
+import net.minecraft.inventory.Inventory;
+import net.minecraft.item.ItemStack;
+
+@Mixin(TrinketInventory.class)
+abstract class MixinTrinketInventory implements Inventory {
+
+    @Override
+    public boolean isValid(int slot, ItemStack stack) {
+        ItemStack existingStack = getStack(slot);
+        SlotReference ref = new SlotReference((TrinketInventory)(Object)this, slot);
+
+
+        int max = Math.min(
+                existingStack.isEmpty() ? 64 : TrinketsDelegateImpl.getMaxCount(existingStack, ref, existingStack.getMaxCount()),
+                stack.isEmpty() ? 64 : TrinketsDelegateImpl.getMaxCount(stack, ref, stack.getMaxCount())
+        );
+
+        int combinedCount = stack.getCount();
+
+        if (ItemStack.canCombine(existingStack, stack)) {
+            combinedCount += existingStack.getCount();
+        }
+
+        return combinedCount <= max;
+    }
+
+}

src/main/resources/unicopia.mixin.json

@@ -36,6 +36,7 @@
     "trinkets.MixinTrinketSurvivalSlot",
     "trinkets.MixinTrinketCreativeSlot",
     "trinkets.MixinTrinketItem",
+    "trinkets.MixinTrinketInventory",
     "trinkets.MixinScreenHandler"
   ],
   "client": [