Keep free measurement preview status

CHANGELOG.md

@@ -9,6 +9,7 @@
 - UI: restrict normal measurement picks and hover previews to the locked measurement target so clicks on neighboring geometry no longer seed selected-object snap endpoints.
 - UI: require the locked measurement target to be the frontmost normal pick before snapping, preventing hidden target geometry behind another part from receiving ruler endpoints.
 - UI: clear stale measurement snap status after cancelling a pending endpoint so the selected-object ruler returns to a neutral ready state.
+- UI: keep the measurement strip in Free pick status while Alt/Option previewing the backup free ruler path, even when the pointer is over empty canvas space.
 - Performance: cache selected-target measurement snap candidates while measuring so hover previews do not repeatedly rebuild vertex and edge lists for the same object.
 - Performance: validate selected-target measurement snap caches against target mesh signatures so transformed or updated geometry rebuilds snap candidates instead of reusing stale vertex and edge lists.
 - UI: restyle distance measurements as orthographic drawing-style dimension callouts with extension lines, arrowheads, and compact drafting labels instead of thick freehand ruler lines.

src/render/babylon/scene.ts

@@ -2290,6 +2290,7 @@ export class BabylonModelPreview implements WorkbenchPreview {
     const y = this.lastPointerClient.y - rect.top;
     let endPoint: Vector3 | null = null;
     if (this.lastPointerClient.altKey) {
+      this.setMeasurementSnapKind("free");
       const pickResult = this.scene.pick(x, y, (mesh) => mesh !== this.previewLine && !this.measurementMarkers.includes(mesh as Mesh));
       endPoint = pickResult.hit && pickResult.pickedPoint
         ? this.resolveMeasurementPickPoint(pickResult.pickedPoint, true)

src/render/three/scene.ts

@@ -3038,6 +3038,7 @@ export class ThreeModelPreview implements WorkbenchPreview {
     this.raycaster.setFromCamera(this.pointer, this.camera);
     let endPoint: Vector3 | null = null;
     if (this.lastPointerClient.altKey) {
+      this.setMeasurementSnapKind("free");
       const hit = this.raycaster.intersectObjects(this.getRenderableObjects(this.rootObject), false)[0];
       endPoint = hit?.point
         ? this.resolveMeasurementPickPoint(hit.point.clone(), true)