[Bug Fix] Selection highlighting breaks in long lists due to Widget Recycling (Missing Keys)

[p5ych0p41h]

Description:
Hello! Thanks for this great library.

I encountered a visual bug when using a dropdownList with many items (e.g., more than 15-20 items). When scrolling down the list, items that are not selected appear with the "selected" style (e.g., grey background), while the actual selected item might lose its highlighting.

The Problem:
This is a classic Flutter ListView recycling issue.

1. DropdownWidget uses ListView.builder.
2. DropdownItemWidget is a StatefulWidget that holds an AnimationController for the selection effect.
3. When scrolling, Flutter recycles the State of widgets that go off-screen to render new items coming on-screen.
4. Since DropdownItemWidget does not have a unique Key, Flutter reuses the State of a previously rendered item. If the previous item was selected (Animation value 1.0), the new item reusing that state will also appear selected, even if item.isSelected is false.

The Fix:
I have implemented a fix that solves this by doing two things:

1. Assigning a unique ValueKey to each DropdownItemWidget so Flutter knows they are different entities.
2. Initializing the animation value in initState to ensure correct rendering when a widget is first built.

Here are the changes needed:

1. lib/widgets/dropdown_widget.dart

In the itemBuilder, add a key:

// ... inside ListView.builder
itemBuilder: (_, index) => GestureDetector(
  onTap: () {
    // ...
  },
  child: Column(
    children: [
      // ...
      DropdownItemWidget(
        // [FIX] Add a unique Key here! 
        // This prevents the State (and AnimationController) from being reused incorrectly.
        key: ValueKey(widget.dropdownList[index].hashCode), 
        
        item: widget.dropdownList[index],
        dropdownItemOptions: widget.dropdownItemOptions,
      ),
      // ...
    ],
  ),
),

2. lib/widgets/dropdown_item_widget.dart

Initialize the controller value in initState. Without this, if you scroll to a pre-selected item that wasn't initially on screen, it might animate from 0 instead of being already highlighted.

class _DropdownItemWidgetState extends State<DropdownItemWidget>
    with SingleTickerProviderStateMixin {
  
  // ... controller definitions ...

  // [FIX] Add initState
  @override
  void initState() {
    super.initState();
    // Immediately set the animation state if the item is already selected
    if (widget.item.isSelected) {
      _controller.value = 1.0;
    }
  }

  // ... dispose and build methods ...
}

With these changes, the dropdown works perfectly with lists of any size (tested with 20+ items).

Hope this helps!

P.S. I'm really looking forward to the library update!