Keep opposite line endpoint fixed during grip edits

src/scene/parametric_constraints.rs

@@ -83,17 +83,17 @@ impl ParametricRef {
     }
 }
 
-/// Point reference driven by a native entity grip. Segment midpoint and
-/// scalar radius grips deliberately return None because they do not identify
-/// one stored point parameter.
-pub(crate) fn driven_ref_for_grip(
+/// Point reference temporarily pinned while solving a native grip edit.
+/// A line endpoint anchors its opposite endpoint so the grabbed endpoint is
+/// projected onto the constraint solution instead of moving the whole line.
+pub(crate) fn grip_solve_anchor_ref(
     entity: &acadrust::EntityType,
     handle: Handle,
     grip_id: usize,
 ) -> Option<ParametricRef> {
     match entity {
         acadrust::EntityType::Line(_) if grip_id <= 1 => {
-            Some(ParametricRef::point(handle, grip_id as i32))
+            Some(ParametricRef::point(handle, 1 - grip_id as i32))
         }
         acadrust::EntityType::LwPolyline(polyline) if grip_id < polyline.vertices.len() => {
             Some(ParametricRef::point(handle, grip_id as i32))
@@ -1888,18 +1888,18 @@ mod tests {
         ));
 
         assert_eq!(
-            driven_ref_for_grip(&arc, handle, 0),
+            grip_solve_anchor_ref(&arc, handle, 0),
             Some(ParametricRef::center(handle))
         );
         assert_eq!(
-            driven_ref_for_grip(&arc, handle, 1),
+            grip_solve_anchor_ref(&arc, handle, 1),
             Some(ParametricRef::point(handle, 0))
         );
         assert_eq!(
-            driven_ref_for_grip(&arc, handle, 2),
+            grip_solve_anchor_ref(&arc, handle, 2),
             Some(ParametricRef::point(handle, 1))
         );
-        assert_eq!(driven_ref_for_grip(&arc, handle, 3), None);
+        assert_eq!(grip_solve_anchor_ref(&arc, handle, 3), None);
     }
 
     #[test]

src/scene/parametric_solve.rs

@@ -1942,10 +1942,10 @@ fn solve_scope(
         sys.add_constraint(Rc::new(Difference::new(el.center.y, el.focus1.y, dy)));
     }
 
-    // The first ordered Coincident selection is the anchor for this solve.
-    // Pin either its addressed point or every defining parameter of its whole
-    // curve temporarily; the relation remains persistent, while these pins do
-    // not enter the drawing's constraint graph.
+    // Temporarily make each requested anchor a fixed kernel input. Marking the
+    // geometry parameters as driven keeps them exact and out of the solver's
+    // free-variable list; an approximate equality equation would still allow
+    // visible drift after repeated grip frames.
     for reference in driven_refs {
         if !set
             .constraints
@@ -1996,9 +1996,7 @@ fn solve_scope(
             }
         };
         for parameter in params_to_pin {
-            let value = sys.store().get(parameter);
-            let target = sys.add_param(value, true);
-            sys.add_constraint(Rc::new(Equal::new(parameter, target, 1.0)));
+            sys.store_mut().set_driven(parameter, true);
         }
     }