feat: Add DropDown UI element to built in UI library

sources/editor/Stride.Assets.Presentation/AssetEditors/UIEditor/Services/UIEditorController.cs

@@ -116,6 +116,8 @@ public override async Task<bool> CreateScene()
                 return false;
 
             rootElements.ForEach(r => RootElements[r.Id] = r);
+            foreach (var root in rootElements)
+                DisableDropDownInteraction(root);
 
             var resolution = uiDesign.Resolution;
             var size = resolution / DesignDensity;
@@ -209,6 +211,7 @@ public override Task AddPart([NotNull] UIHierarchyItemViewModel parent, UIElemen
             EnsureAssetAccess();
 
             var gameSidePart = ClonePartForGameSide(parent.Asset.Asset, assetSidePart);
+            DisableDropDownInteraction(gameSidePart);
             return InvokeAsync(() =>
             {
                 Logger.Debug($"Adding element {assetSidePart.Id} to game-side scene");
@@ -424,6 +427,28 @@ private UIComponent GetUIComponent()
             return GetEntityByName(UIEntityName)?.Get<UIComponent>();
         }
 
+        /// <summary>
+        /// Recursively traverses the visual tree rooted at <paramref name="element"/> and disables
+        /// popup interaction on every <see cref="DropDown"/> found.
+        /// </summary>
+        /// <remarks>
+        /// In the editor, <see cref="DropDown"/> popups open but render only the background — item text
+        /// is not visible. This is a workaround that prevents the popup from opening so the control
+        /// remains in a consistent state during editing. Runtime behavior is unaffected.
+        /// </remarks>
+        /// <param name="element">The root of the visual sub-tree to search. Must not be <see langword="null"/>.</param>
+        private static void DisableDropDownInteraction(UIElement element)
+        {
+            if (element is DropDown dropDown)
+            {
+                dropDown.IsInteractable = false;
+                return;
+            }
+            var children = element.VisualChildren;
+            for (var i = 0; i < children.Count; i++)
+                DisableDropDownInteraction(children[i]);
+        }
+
         /// <summary>
         /// Iterates through the visual tree of <paramref name="element"/> and returns the first element matching the <paramref name="elementId"/>.
         /// </summary>

sources/engine/Stride.Engine/AssetPackage/Assets/Shared/StrideUILibrary.sduilib

@@ -11,6 +11,7 @@ PublicUIElements:
     6a778e64-1d79-44c8-b209-be1779349abb: TextBlock
     72d195ac-472a-4f9b-8540-9f4f7d5fc53e: Grid
     7d51286a-f844-48de-afe1-cb6b00e2dcb8: ScrollingText
+    8fa7caaf-4e9c-4efb-80ad-58f7f61c25e0: DropDown
     97a0fc3a-e627-4122-b585-d33a6a7cd785: StackPanel
     9b996723-bbd3-44c3-9406-fdc49875d2a2: Canvas
     a35da784-9d42-4fe0-bd28-3eb547bc2812: ContentDecorator
@@ -25,6 +26,7 @@ Hierarchy:
     RootParts:
         - !Button ref!! e8985e3a-8d82-40a6-adb1-76619eb19cd8
         - !EditText ref!! eef44bd6-bd2d-4abe-8335-0971d5d83b10
+        - !DropDown ref!! 8fa7caaf-4e9c-4efb-80ad-58f7f61c25e0
         - !Slider ref!! fcd540ef-6f9e-4cab-b24d-ba1533b0fdb9
         - !ToggleButton ref!! 1dfd7e9f-ab5a-4849-b6e3-1c50b0974f5f
         - !Border ref!! 606486b0-61b1-498a-beed-5425893fc384
@@ -120,6 +122,42 @@ Hierarchy:
                 Text: 'Scrolling text! '
                 Font: c90f3988-0544-4cbe-993f-13af7d9c23c6:StrideDefaultFont
                 TextColor: {R: 240, G: 240, B: 240, A: 255}
+        -   UIElement: !DropDown
+                Id: 8fa7caaf-4e9c-4efb-80ad-58f7f61c25e0
+                DependencyProperties: {}
+                BackgroundColor: {R: 0, G: 0, B: 0, A: 0}
+                DrawLayerNumber: 3
+                CanBeHitByUser: true
+                Margin: {}
+                Name: DropDown
+                Padding: {}
+                OpenedImage: !SpriteFromSheet
+                    Sheet: 1be3340c-b797-4557-bff7-d9be92dfdb42:StrideUIDesigns
+                    CurrentFrame: 2
+                ClosedImage: !SpriteFromSheet
+                    Sheet: 1be3340c-b797-4557-bff7-d9be92dfdb42:StrideUIDesigns
+                    CurrentFrame: 1
+                MouseOverImage: !SpriteFromSheet
+                    Sheet: 1be3340c-b797-4557-bff7-d9be92dfdb42:StrideUIDesigns
+                Font: c90f3988-0544-4cbe-993f-13af7d9c23c6:StrideDefaultFont
+                TextSize: 20.0
+                TextColor: {R: 240, G: 240, B: 240, A: 255}
+                ListBackground: !SpriteFromSheet
+                    Sheet: 1be3340c-b797-4557-bff7-d9be92dfdb42:StrideUIDesigns
+                    CurrentFrame: 4
+                ListItemNotPressedImage: !SpriteFromSheet
+                    Sheet: 1be3340c-b797-4557-bff7-d9be92dfdb42:StrideUIDesigns
+                    CurrentFrame: 1
+                ListItemPressedImage: !SpriteFromSheet
+                    Sheet: 1be3340c-b797-4557-bff7-d9be92dfdb42:StrideUIDesigns
+                    CurrentFrame: 2
+                ListItemMouseOverImage: !SpriteFromSheet
+                    Sheet: 1be3340c-b797-4557-bff7-d9be92dfdb42:StrideUIDesigns
+                ItemTextColor: {R: 240, G: 240, B: 240, A: 255}
+                ItemTextSize: 20.0
+                ScrollBarColor: {R: 25, G: 25, B: 25, A: 255}
+                PlaceholderText: Select an option
+                Items: {}
         -   UIElement: !StackPanel
                 Id: 97a0fc3a-e627-4122-b585-d33a6a7cd785
                 DependencyProperties: {}