Reduces redundant clamping on labels

Author: mzschwartz5

packages/engine/Source/Scene/LabelCollection.js

@@ -11,7 +11,7 @@ import bitmapSDF from "bitmap-sdf";
 import BillboardCollection from "./BillboardCollection.js";
 import BillboardTexture from "./BillboardTexture.js";
 import BlendOption from "./BlendOption.js";
-import { isHeightReferenceClamp } from "./HeightReference.js";
+import HeightReference, { isHeightReferenceClamp } from "./HeightReference.js";
 import HorizontalOrigin from "./HorizontalOrigin.js";
 import Label from "./Label.js";
 import LabelStyle from "./LabelStyle.js";
@@ -306,7 +306,8 @@ function rebindAllGlyphs(labelCollection, label) {
     billboard.pixelOffset = label._pixelOffset;
     billboard.horizontalOrigin = HorizontalOrigin.LEFT;
     billboard.verticalOrigin = label._verticalOrigin;
-    billboard.heightReference = label._heightReference;
+    // To avoid excessive height update listeners, glyphs are not clamped. Their height values are updated manually by the owning label.
+    billboard.heightReference = HeightReference.NONE;
     billboard.scale = label.totalScale;
     billboard.pickPrimitive = label;
     billboard.id = label._id;

packages/engine/Specs/Scene/LabelCollectionSpec.js

@@ -2446,7 +2446,8 @@ describe("Scene/LabelCollection", function () {
           });
 
           expect(l._clampedPosition).toBeDefined();
-          expect(l._glyphs[0].billboard._clampedPosition).toBeDefined();
+          // Glyphs are not truly clamped (to avoid excessive listeners and globe picks). They simply copy their owning label's clamped position.
+          expect(l._glyphs[0].billboard._clampedPosition).toBeUndefined();
 
           l.heightReference = HeightReference.NONE;
           expect(l._clampedPosition).toBeUndefined();
@@ -2459,13 +2460,14 @@ describe("Scene/LabelCollection", function () {
             text: "t",
             position: Cartesian3.fromDegrees(-72.0, 40.0),
           });
+          l.showBackground = true;
 
           await pollToPromise(() => {
             scene.renderForSpecs();
             return labelsWithHeight.ready;
           });
 
-          const billboard = l._glyphs[0].billboard;
+          const billboard = l._backgroundBillboard;
           expect(billboard._removeCallbackFunc).toBeDefined();
           const spy = spyOn(billboard, "_removeCallbackFunc");
           labelsWithHeight.remove(l);