Sketch where the user pointed: a picked face is the sketch plane

Selecting a face and sketching on it is the most common gesture in solid
modelling, and it was impossible. The plane came from a combo holding
XY/XZ/YZ plus datums, and plane_from_choice had no face branch at all, so
the only route onto a face was to build a Coincident datum plane on it
first, confirm that, reopen the sketch and find the datum in the
dropdown. Three extra steps and a junk feature in the tree.

The fix is not another combo row. A new sketch now takes its plane from
what is SELECTED IN THE VIEWPORT: a picked planar face wins outright, and
only when nothing is picked does it fall back to the reference plane —
which is itself normally set by clicking one of the ghost planes in 3D,
not by opening the combo. The tool hint names the target ("Circle — click
center, then radius · on the picked face") so the choice is visible on the
geometry side rather than needing a control to read back.

CadDocument::plane_of_face is the shared derivation, so the sketch path
and the Coincident datum method cannot drift apart. It refuses
non-planar faces: face_normal_world evaluates at the mid parameter, which
on a cylinder or a fillet is a tangent plane at one arbitrary point —
fine for offsetting a datum, wrong as a sketch plane, and silently
sketching on a tangent is worse than declining.

Picking the face also CONSUMES it. Leaving the pick live meant the next
Extrude saw a selected face and push/pulled it instead of extruding the
sketch just drawn — the same trap the imported-art path already guards
against.

Verified on :10 end to end with no combo interaction: build a box, click
its top face twice to cycle whole -> face, press S then C, and the circle
is drawn in the plane of that face with its Radius tab on the geometry
(artifacts/shots/f3a2-03-face.png, f3a2-04-sketch-on-face.png,
f3a2-05-circle-drawn.png). Kernel side: 154 cases / 2125 assertions green
on both forks, including that a cylinder resolves exactly its two flat
caps and refuses the barrel.

Still side-panel-shaped and to be dealt with separately: the Plane combo
remains on the card and now merely displays a stale row when a face is
the real target. It should show the actual target or go away.

snaporca-3a2.

src/slic3r/GUI/DesignPanel.cpp

@@ -341,16 +341,24 @@ DesignPanel::DesignPanel(wxWindow* parent)
     auto select_tool = [this](DesignSketchTool::Mode mode, const wxString& hint) {
         if (!m_viewport) return;
         if (!m_viewport->is_sketching()) {
-            const SketchPlane plane = plane_from_choice(m_draw_plane->GetSelection());
+            wxString on;
+            const SketchPlane plane = sketch_plane_from_selection(on);
             m_viewport->begin_sketch(plane, mode);
             m_construction->SetValue(false);   // a fresh session starts non-construction
+            m_sketch_on = on;                  // shown with the tool hint, so the target is visible
+            // The face has been CONSUMED as the sketch plane, so drop the pick. Leaving it live
+            // meant the next Extrude saw a selected face and push/pulled it instead of extruding
+            // the sketch just drawn — the same trap the imported-art path already guards against.
+            if (m_sel_solid_face >= 0)
+                m_sel_solid_face = m_sel_solid_edge = m_sel_solid_body = -1;
         } else {
             m_viewport->set_sketch_tool(mode);
         }
         m_viewport->set_sketch_construction(m_construction->GetValue());
         show_polygon_card(mode == DesignSketchTool::Mode::Polygon);
         m_status->SetForegroundColour(wxNullColour);
-        m_status->SetLabel(hint);
+        m_status->SetLabel(m_sketch_on.IsEmpty() ? hint
+                           : wxString::Format(_L("%s  ·  on %s"), hint, m_sketch_on));
         m_status->Refresh();
     };
 
@@ -3523,6 +3531,7 @@ void DesignPanel::set_active_tool_btn(ScalableButton* b)
 void DesignPanel::set_ui_mode(UiMode m)
 {
     m_ui_mode = m;
+    if (m != UiMode::Sketch) m_sketch_on.clear();   // no stale "on the picked face" on the next hint
     wxSizer* s = m_toolbar->GetSizer();
     s->Show(m_tb_feature,   m == UiMode::Feature,   true);
     s->Show(m_tb_sketch,    m == UiMode::Sketch,    true);
@@ -4837,6 +4846,29 @@ SketchPlane DesignPanel::plane_from_choice(int row) const
     SketchPlane p = SketchPlane::XY(); p.origin += m_doc.modeling_origin; return p;
 }
 
+// A sketch goes where the user pointed. A planar face picked in the viewport wins outright; only
+// when nothing is picked do we fall back to the reference plane, which itself is normally set by
+// clicking one of the ghost planes in 3D (on_datum_base_picked) rather than by opening the combo.
+// Before this, a picked face was ignored and the only way onto it was to build a Coincident datum
+// plane first and then find it in a dropdown — three steps and a junk feature in the tree for the
+// most common gesture in solid modelling. snaporca-3a2.
+SketchPlane DesignPanel::sketch_plane_from_selection(wxString& what) const
+{
+    SketchPlane p;
+    if (m_sel_solid_face >= 0 && m_sel_solid_body >= 0
+        && m_doc.plane_of_face(m_sel_solid_body, m_sel_solid_face, p)) {
+        what = (m_doc.bodies.size() > 1)
+             ? wxString::Format(_L("the picked face of Body %d"), m_sel_solid_body + 1)
+             : _L("the picked face");
+        return p;
+    }
+    const int row = m_draw_plane ? m_draw_plane->GetSelection() : 0;
+    what = (m_draw_plane && row >= 0 && row < int(m_draw_plane->GetCount()))
+         ? m_draw_plane->GetString(unsigned(row))
+         : wxString("XY");
+    return plane_from_choice(row);
+}
+
 void DesignPanel::apply_plane_refs(CadFeature& f) const
 {
     f.plane_type       = (PlaneType)m_plane_type->GetSelection();