UIElements block input to TextField

I'm trying to use the new UIElements API for an editor tool, and i'd like to selectively block key-input. For example i dont want the user to be able to input any space character.

In the old UI i'd listen to an KeyDown event type and use it, in UIElements i think i have to register an KeyDownEvent callback and call StopPropagation. That doesnt seem to work, tried this via an Manipulator too to no avail. Tried various Capture modes, and StopImmediatePropegation.

For example:

Code (csharp):

1.  
2.  
3.     void OnEnable()
4.     {
5.         var root = this.GetRootVisualContainer();
6.  
7.         root.AddStyleSheetPath("styles");
8.         var searchBox = new TextField()
9.         {
10.             name = "searchBox"
11.         };
12.         searchBox.RegisterCallback<KeyDownEvent>(onKeyDown, Capture.Capture);
13.  
14.         searchBox.OnTextChanged += onSearchChanged;
15.         root.Add(searchBox);
16.     }
17.  
18.     private void onKeyDown(KeyDownEvent evt)
19.     {
20.         if(evt.keyCode == KeyCode.Space)
21.         {
22.             evt.imguiEvent.Use();
23.             evt.StopPropagation();
24.             evt.StopImmediatePropagation();
25.         }
26.     }
27.  
28.  

Still allows me to insert a space. What am i doing wrong?