DropdownMenu with right click?

That's awesome to hear! I'll be curious to see the final result.

As for the menu, here's how I would do it:

Code (CSharp):

1. void OnEnable()
2. {
3.     ...
4.     elementToRightClickOn.RegisterCallback<MouseUpEvent>(HandleRightClick);
5.     ...
6. }
7.  
8. private void HandleRightClick(MouseUpEvent evt)
9. {
10.     if (evt.button != (int)MouseButton.RightMouse)
11.         return;
12.  
13.     var targetElement = evt.target as VisualElement;
14.     if (targetElement == null)
15.         return;
16.  
17.     var menu = new GenericMenu();
18.  
19.     int menuItemValue = 5;
20.  
21.     // Add a single menu item
22.     bool isSelected = true;
23.     menu.AddItem(new GUIContent("some menu item name"), isSelected,
24.         value => ChangeValueFromMenu(value),
25.         menuItemValue);
26.  
27.     // Get position of menu on top of target element.
28.     var menuPosition = new Vector2(targetElement.layout.xMin, targetElement.layout.height);
29.     menuPosition = this.LocalToWorld(menuPosition);
30.     var menuRect = new Rect(menuPosition, Vector2.zero);
31.  
32.     menu.DropDown(menuRect);
33. }
34.  
35. private void ChangeValueFromMenu(object menuItem)
36. {
37.     doSomethingWithValue(menuItem as int);
38. }

But your MouseManipulator solution also works for invoking the menu.

 

The GenericMenu has not been a focus for the initial release of UIElements and no immediate plans around it right now. It is something that might be addressed as part of the reskin effort in the future.

That said, it's fairly straightforward to implement your own using a Popup-style EditorWindow and UIElements events. Here's some random snippet of code I found that brings up an custom EditorWindow where the mouse is in another window (the Host window) - might need some tweaking but should work:

Code (CSharp):

1. class HostWindow : EditorWindow
2. {
3.     void OnEnable()
4.     {
5.         rootVisualElement.RegisterCallback<MouseDownEvent>(OnMouseDown);
6.     }
7.  
8.     void OnMouseDown(MouseDownEvent evt)
9.     {
10.         if (evt.button == 1) // Right click
11.             CustomMenuWindow.Show(this, evt.mousePosition);
12.     }
13. }
14.  
15. class CustomMenuWindow : EditorWindow
16. {
17.     public static void Show(
18.         EditorWindow host,
19.         Vector2 displayPosition)
20.     {
21.         var window = CreateInstance<CustomMenuWindow>();
22.         var position = GetPosition(host, displayPosition);
23.         window.position = new Rect(position + host.position.position, s_DefaultSize);
24.         window.ShowPopup();
25.         window.Focus();
26.     }
27.  
28.     private static Vector2 GetPosition(EditorWindow host, Vector2 displayPosition)
29.     {
30.         var x = displayPosition.x;
31.         var y = displayPosition.y;
32.  
33.         // Searcher overlaps with the right boundary.
34.         if (x + s_DefaultSize.x >= host.position.size.x)
35.             x -= s_DefaultSize.x;
36.  
37.         // The displayPosition should be in window world space but the
38.         // EditorWindow.position is actually the GetRootVisualContainer()
39.         // rectangle, not including the tabs area. So we need to do a
40.         // small correction here.
41.         y -= host.GetRootVisualContainer().style.positionTop;
42.  
43.         // Searcher overlaps with the bottom boundary.
44.         if (y + s_DefaultSize.y >= host.position.size.y)
45.             y -= s_DefaultSize.y;
46.  
47.         return new Vector2(x, y);
48.     }
49. }

 

It's worth adding that for bringing up system contextual menus we do have a Manipulator and Event in place. Here's some example usage:

Code (CSharp):

1. using UnityEditor;
2. using UnityEngine;
3. using UnityEngine.UIElements;
4.  
5. public class TestWindow : EditorWindow
6. {
7.     [MenuItem("Example/ContextMenu")]
8.     static void Bla() {
9.         GetWindow<TestWindow>();
10.     }
11.  
12.     void OnEnable()
13.     {
14.         var button = new Button() { text = "Menu" };
15.         button.AddManipulator(new ContextualMenuManipulator((evt) =>
16.         {
17.             evt.menu.AppendAction("Custom Action 1",
18.                 (a) => Debug.Log(a.userData as string),
19.                 (a) => DropdownMenuAction.Status.Normal,
20.                 "ACTION 1");
21.             evt.menu.AppendAction("Custom Action 2",
22.                 (a) => Debug.Log(a.userData as string),
23.                 (a) => DropdownMenuAction.Status.Normal,
24.                 "ACTION 2");
25.         }));
26.  
27.         rootVisualElement.Add(button);
28.     }
29. }