Fix USD camera orbit pivot — use scene bbox centroid instead of focus distance

When switching to a USD camera, the orbit pivot was set from
GfCamera::GetFocusDistance() which is a depth-of-field attribute
that defaults to 5 scene units regardless of scene scale.

InitCameraNavigation() now computes the scene bounding box centroid,
projects it onto the camera view ray to get a proper orbit distance,
and calls SetFocalPoint/SetDist to place the pivot correctly.
Added SetDist() setter on ViewportCamera to support this.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/core/ViewportCamera.h

@@ -113,6 +113,14 @@ public:
         m_cameraTransformDirty = true;
     }
 
+    /// Set the orbit distance directly.
+    /// Used when initialising from a USD camera prim to override the camera's
+    /// authored focus distance (which is a DOF attribute, not an orbit radius).
+    void SetDist(double d) {
+        m_dist = d;
+        m_cameraTransformDirty = true;
+    }
+
     /// Returns the current near-clip distance.
     double GetNearClip() const {
         return static_cast<double>(m_camera.GetClippingRange().GetMin());

src/ui/ViewportTile.cpp

@@ -120,6 +120,33 @@ void ViewportTile::InitCameraNavigation()
     pxr::UsdGeomCamera usdCam(prim);
     pxr::GfCamera gfCam = usdCam.GetCamera(m_displayTime);
     m_camera.SwitchToFreeCamera(&gfCam);
+
+    // PullFromCameraTransform() sets m_dist from the USD camera's focus-distance
+    // attribute, which is a depth-of-field parameter (default 5 scene units) —
+    // not the distance to scene content.  Override both the orbit center and
+    // distance using the scene bounding box so tumbling pivots on visible geometry.
+    {
+        pxr::TfTokenVector purposes = {
+            pxr::UsdGeomTokens->default_, pxr::UsdGeomTokens->proxy };
+        pxr::UsdGeomBBoxCache bboxCache(m_displayTime, purposes, true);
+        pxr::GfBBox3d stageBBox =
+            bboxCache.ComputeWorldBound(m_stage->GetPseudoRoot());
+        if (!stageBBox.GetRange().IsEmpty()) {
+            pxr::GfVec3d sceneCenter = stageBBox.ComputeCentroid();
+            pxr::GfVec3d camEye      = gfCam.GetFrustum().GetPosition();
+            pxr::GfVec3d viewDir     = gfCam.GetFrustum().ComputeViewDirection();
+
+            // Project the scene centre onto the view ray to get the orbit
+            // distance; this keeps the camera at its current position while
+            // placing the pivot at the closest on-axis approach to the scene.
+            double dist = pxr::GfDot(sceneCenter - camEye, viewDir);
+            if (dist > 0.01) {
+                m_camera.SetFocalPoint(camEye + dist * viewDir);
+                m_camera.SetDist(dist);
+            }
+        }
+    }
+
     m_isDrivingUsdCamPrim = true;
     m_drivenUsdCamPath    = camPath;
 }