C# Lists & Inventory Logic

I'm working on my Drag and Drop inventory and nearly have a working prototype but my logic is off somewhere in managing the Inventory list and correspondong bagSlot and after numerous attempts figured I would ask for some help.

Update: After posting original code, I fixed my logic errors and now just chasing down last one. If I take an item from slot X and move it into a slot X + (some higher slot), I haveto click into the new slot twice. If I take an item from slot X and move it into a slot lower#, it works as intended registering the mouseUP and drops the icon.

I'm new to Lists, LINQ and OOP in general so I could be misusing stuff or just flubbed some logic.

TL;DR: what's wrong with my ItemDrop approach?

  private void InventoryWindow(int WinID) {
 
         // Increments from 0...invWinRows * invWinCols 
         int cnt = 0;  
 
         bool slotUsed = false;
 
         // Display rows and cols in Window
         for (int y=0; y < invWinRows; y++) {
             for(int x=0; x < invWinCols; x++) {
 
                 // Build Rect/Bag Slot
                 float xPos = 5+(x*iconDim);
                 float yPos = 20+(y*iconDim);
                 invSlotRect = new Rect(xPos,yPos,iconDim,iconDim);
 
                 // LINQ: look through Inventory and see if cnt/current invBagSlot is a match
                 // returns -1 if it is not;  UA @KellyThomas - danka!
                 int ItemINDEX = pinv.pInventory.IndexOf(pinv.pInventory.Where (i => i.invBagSlot == cnt).FirstOrDefault ());
 
                 if(ItemINDEX == -1) 
                     slotUsed = false;
                 else 
                     slotUsed = true;
 
 
                 if(invSlotRect.Contains(Event.current.mousePosition)) {
 
                     //    Debug.Log("Mouse in box " + cnt);
 
                     // ********* Right click to Use item ********* //
                     if(slotUsed && Event.current.type == EventType.MouseDown && Event.current.button == 1 && bagSlotState == BagSlotState.None) {
 
                         Debug.Log (pinv.pInventory[cnt].invName + "was right-clicked");
                         Debug.Log (pinv.pInventory[cnt].invID + " is the ID");
                     //    cursorActive = CursorState.Farming;
                         bagSlotState = BagSlotState.UsingItem;
                         activeItemListIDX = cnt;
                         activeItem = true;
                     }
 
 
                     // *********** Left click to pick up item *********//
                     if(Event.current.type == EventType.MouseUp && Event.current.button == 0 && slotUsed && bagSlotState == BagSlotState.None) {
 
                         ItemINDEX = pinv.pInventory.IndexOf(pinv.pInventory.Where (i => i.invBagSlot == cnt).FirstOrDefault ());
                     //    dragclass.pic = pinv.pInventory[ItemINDEX].invIconTexture;
                         dragclass.SetIconProps(pinv.pInventory[ItemINDEX].invIconTexture, pinv.pInventory[ItemINDEX].invStackCount);
                         itemInMotion = ItemINDEX;
 
         Debug.Log ("PICKUP ==>  Inv Slot: " + cnt + " Item: " + pinv.pInventory[itemInMotion].invName + " INDEX: " + ItemINDEX);
 
                         //bagSlotState = BagSlotState.PickingUpItem;
                         bagSlotState = BagSlotState.DraggingItem;
 
                     }
 
                     // *********** Left click held down, dragging item ********//
                 //    if((Event.current.type == EventType.mouseDrag || Event.current.type == EventType.MouseDown) && bagSlotState == BagSlotState.PickingUpItem) {
 
 
     if(Event.current.type == EventType.mouseDrag && bagSlotState == BagSlotState.PickingUpItem) {
 
                             bagSlotState = BagSlotState.DraggingItem;
     
 
                     }
 
                     // ************* Left click, dropping item into unused inventory slot *************//    
                     if(Event.current.type == EventType.MouseUp && Event.current.button == 0 && !slotUsed && bagSlotState == BagSlotState.DraggingItem) {
         Debug.Log("DROP ==> Inv slot: " +cnt+" Item in Motion: " + pinv.pInventory[itemInMotion].invName + " INDEX: " + itemInMotion);
                         bagSlotState = BagSlotState.DroppingItem;
 
                         pinv.pInventory[itemInMotion].invBagSlot = cnt;
 
                         //dragclass.pic = null;
 
                         dragclass.SetIconProps(null,-1);
 
                         itemInMotion = -1;
                     }
 
             
 
                         // If Event.current is a MouseUp in an occupied slot and we are dragging, that's a Swap
                         // cases to consider later: 1/stackables and on-split stuff, 2/ drop into same box
                         
                         if(Event.current.type == EventType.MouseDown && Event.current.button == 0 && bagSlotState == BagSlotState.DraggingItem && slotUsed == true) {
         Debug.Log ("SWAP ==> Inv SOURCE slot: "+pinv.pInventory[itemInMotion].invBagSlot+ " Item in Motion: " + pinv.pInventory[itemInMotion].invName + " INDEX: "+itemInMotion);
         
                         // Get ListID of new item in cnt/current slot
                         int ItemINDEX2 = pinv.pInventory.IndexOf(pinv.pInventory.Where (i => i.invBagSlot == cnt).FirstOrDefault ());
 
                         // Place the item we were dragging into this slot - but now we have 2 items sharing this slot!
                         pinv.pInventory[ItemINDEX2].invBagSlot = -1;        // move it out
                         pinv.pInventory[itemInMotion].invBagSlot = cnt;        // place the item we are dragging into it
 
                         // Update icon of item in bag
                         // pinv.pInventory[itemInMotion].invIconTexture = pinv.pInventory[ItemINDEX2].invIconTexture;
 
                         // Assign new dragging item
                         itemInMotion = ItemINDEX2;
 
                         // Change icon of dragging object
                         //dragclass.pic = pinv.pInventory[itemInMotion].invIconTexture;
                         dragclass.SetIconProps(pinv.pInventory[ItemINDEX].invIconTexture, pinv.pInventory[ItemINDEX].invStackCount);
                         ItemINDEX = ItemINDEX2;
                     
                 Debug.Log ("SWAP ==> Inv TARGET slot: "+cnt+" Item to Swap "+pinv.pInventory[itemInMotion].invName);
 
                         }
                         
 
                     }
 
         
 
             //    ItemINDEX = pinv.pInventory.IndexOf(pinv.pInventory.Where (i => i.invBagSlot == cnt).FirstOrDefault ());
 
                 if(ItemINDEX != -1) {
                     GUI.Button (new Rect(invSlotRect),pinv.pInventory[ItemINDEX].invIconTexture);
                 //    GUI.Button (new Rect(invSlotRect),qmload.allIcons[IconINDEX].iconTexture);
                 
                     // If item is stackable, display current stack count
                         if(pinv.pInventory[ItemINDEX].invStackable) 
                                 GUI.Label(new Rect(invSlotRect),pinv.pInventory[ItemINDEX].invStackCount.ToString());
                     }
                                 
             else {
 
                     // This is an empty slot
                     GUI.Button(new Rect(invSlotRect),"");
                     }
 
 
                 cnt++;
 
                 if(bagSlotState == BagSlotState.DroppingItem)
                     bagSlotState = BagSlotState.None;
 
             } // x
         } // y, forloop
 
     
         // "X" to close too    
         if (GUI.Button (new Rect ((invWinCols*iconDim)-16, 2, 20, 18), "x"))
                         showInventory = false;
         
         //Empty DragWindow allows dragging if nothing else has precedence/focus
         GUI.DragWindow ();
     }

Dunno if it matters but this is all going on inside a GUI.Window