Draw Unity GL under UI

There are a few possibilities of how you can use this.
1. ImmediateModeElement. You can place your GL calls under the overridden ImmediateRepaint method.
2. You can also generate a custom mesh with VisualElement.generateVisualContent and MeshGenerationContext

 

Using the onGenerateUIContent delegate, you can generate geometry that will be used later when rendering. This has the benefit of allowing batches and reducing drawcalls.

Documentation is a bit lacking at the moment, but here is a sample:

Code (CSharp):

1. class SolidQuad : VisualElement
2.         {
3.             public SolidQuad() { generateVisualContent += OnGenerateVisualContent; }
4.  
5.             static readonly Vertex[] k_Vertices = new Vertex[4];
6.             static readonly ushort[] k_Indices = { 0, 1, 2, 2, 3, 0 };
7.  
8.             void OnGenerateVisualContent(MeshGenerationContext mgc)
9.             {
10.                 Rect r = contentRect;
11.                 if (r.width < 0.01f || r.height < 0.01f)
12.                     return; // Skip rendering when too small.
13.  
14.                 Color color = resolvedStyle.color;
15.                 k_Vertices[0].tint = Color.black;
16.                 k_Vertices[1].tint = Color.black;
17.                 k_Vertices[2].tint = color;
18.                 k_Vertices[3].tint = color;
19.  
20.                 float left = 0;
21.                 float right = r.width;
22.                 float top = 0;
23.                 float bottom = r.height;
24.  
25.                 k_Vertices[0].position = new Vector3(left, bottom, Vertex.nearZ);
26.                 k_Vertices[1].position = new Vector3(left, top, Vertex.nearZ);
27.                 k_Vertices[2].position = new Vector3(right, top, Vertex.nearZ);
28.                 k_Vertices[3].position = new Vector3(right, bottom, Vertex.nearZ);
29.  
30.                 MeshWriteData mwd = mgc.Allocate(k_Vertices.Length, k_Indices.Length);
31.                 mwd.SetAllVertices(k_Vertices);
32.                 mwd.SetAllIndices(k_Indices);
33.             }
34.         }

 

You can have a look at this forum post:
https://forum.unity.com/threads/place-text-in-front-of-procedural-mesh.1120159/#post-7203388

In short, right now, you'll need to nest your label inside another VisualElement and set your generateVisualContent on that parent.