Size the Design move/rotate gizmo from the body, like Orca's Prepare gizmos

The Design gizmo already had both move arrows and rotation rings, but drew them
at a fixed 70 px arm regardless of the body: on a 40 mm cube everything
collapsed into a ~100 px tangle buried inside the solid, so the rings were
effectively invisible and the tool read as "move only, no rotate".

Orca's Prepare gizmos size themselves from the selection's bounding sphere
(GLGizmoRotate3D: m_radius = Offset + sphere radius) so the handles always clear
the object. Same rule here: DesignPanel passes the body's bounding-sphere radius
(scale-aware) into the gizmo, and move_gizmo_arm() returns
max(70 px, 1.25 * radius) — the screen-space floor keeps it grabbable on a tiny
body or when zoomed far out. Ring radius and BOTH hit-tests derive from that one
helper, so picking cannot drift from what is drawn.

Verified on a 40 mm cube: rings now encircle the body, arrow drag moves with a
live mm readout, ring drag rotates live.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BVzKmX6Y1aEteit1HTXG4Q

src/slic3r/GUI/DesignSketchTool.cpp

@@ -3390,7 +3390,8 @@ int DesignSketchTool::hit_test_base_pick(GLCanvas3D& canvas, const wxMouseEvent&
 }
 
 // ---- Move-body gizmo (M5) -------------------------------------------------------------
-void DesignSketchTool::set_move_gizmo(int body, const Vec3d& pivot, const Transform3d& base_xform)
+void DesignSketchTool::set_move_gizmo(int body, const Vec3d& pivot, const Transform3d& base_xform,
+                                      double body_radius)
 {
     m_mv_active     = true;
     m_mv_body       = body;
@@ -3399,6 +3400,19 @@ void DesignSketchTool::set_move_gizmo(int body, const Vec3d& pivot, const Transf
     m_mv_offset     = Vec3d::Zero();
     m_mv_rot        = Eigen::Matrix3d::Identity();
     m_mv_drag       = -1;
+    m_mv_radius     = std::max(body_radius, 0.0);
+}
+
+// Gizmo arm length in world mm. Orca's Prepare gizmos size themselves from the selection's
+// bounding sphere (GLGizmoRotate3D: m_radius = Offset + sphere radius) so the handles always sit
+// clear of the object; a fixed screen-size arm instead collapsed into a tangle buried inside a
+// large solid, which is why the rotation rings read as "missing". Same idea here, with a
+// screen-space floor so the gizmo stays grabbable on a tiny body or when zoomed far out.
+double DesignSketchTool::move_gizmo_arm(const Camera& cam) const
+{
+    const double upp = 1.0 / std::max(cam.get_zoom(), 1e-6);
+    const double screen_min = 70.0 * upp;              // never smaller than the old fixed size
+    return std::max(screen_min, m_mv_radius * 1.25);   // 25% clear of the body surface
 }
 
 void DesignSketchTool::clear_move_gizmo()
@@ -3443,7 +3457,7 @@ void DesignSketchTool::render_move_gizmo()
     const Vec3d anchor = m_mv_base + m_mv_offset;
     const double upp  = 1.0 / std::max(cam.get_zoom(), 1e-6);
     const double th   = std::max(15.0 * upp, 1e-4);
-    const double L    = 70.0 * upp;                 // fixed screen-size arrow length
+    const double L    = move_gizmo_arm(cam);        // scales with the body (see move_gizmo_arm)
 
     const SketchPlane saved = m_plane;
     SketchPlane bb; bb.origin = anchor; bb.x_axis = right; bb.y_axis = up; bb.normal = fwd;
@@ -3478,7 +3492,7 @@ void DesignSketchTool::render_move_gizmo()
 
     // Three world-axis rotation rings (X/Y/Z), each a circle in the plane perpendicular to
     // its axis through the gizmo anchor — drag a ring to rotate the body about that axis.
-    const double R = 58.0 * upp;
+    const double R = 0.83 * move_gizmo_arm(cam);   // rings just inside the arrow tips
     for (int a = 0; a < 3; ++a) {
         Vec3d e, u, v; ring_basis(a, e, u, v);
         SketchPlane rp; rp.origin = anchor; rp.x_axis = u; rp.y_axis = v; rp.normal = e;
@@ -3504,7 +3518,7 @@ bool DesignSketchTool::hit_test_move_arrow(GLCanvas3D& canvas, const wxMouseEven
     const Vec3d ro = r.a, rd = r.b - r.a;
     const Camera& cam = wxGetApp().plater()->get_camera();
     const double upp = 1.0 / std::max(cam.get_zoom(), 1e-6);
-    const double L   = 70.0 * upp;
+    const double L   = move_gizmo_arm(cam);         // must match render_move_gizmo
     const Vec3d anchor = m_mv_base + m_mv_offset;
     const Vec3d axes[3] = { Vec3d::UnitX(), Vec3d::UnitY(), Vec3d::UnitZ() };
     int best = -1; double bestd = 7.0 * upp;        // ~7 px tolerance
@@ -3552,7 +3566,7 @@ bool DesignSketchTool::hit_test_move_arc(GLCanvas3D& canvas, const wxMouseEvent&
     const Vec3d ro = r.a, rd = r.b - r.a;
     const Camera& cam = wxGetApp().plater()->get_camera();
     const double upp = 1.0 / std::max(cam.get_zoom(), 1e-6);
-    const double R = 58.0 * upp;
+    const double R = 0.83 * move_gizmo_arm(cam);   // rings just inside the arrow tips
     const Vec3d anchor = m_mv_base + m_mv_offset;
     int best = -1; double bestd = 7.0 * upp;
     const int N = 48;