Sketch symmetry: the free symmetry line moves when both mirrored lines are fixed (#1796)

A constraint is applied with its first-picked entities held and the second
moving; when the second can't move, a first one moves instead. That fallback
was skipped as soon as ANY first entity was locked, so a free construction
line between two fixed lines was never tried, and Symmetry was refused with
"neither entity can move".

Each first entity that can move is now tried on its own, after the second.
Nothing changes where the second can move.

crates/engine/src/cmd/sketch.rs

@@ -1463,10 +1463,10 @@ pub(super) fn apply_in_order(sk: &mut Sketch, rep: &SolveReport, first: &[Ent],
     if !second_locked {
         ordered(first, second);
     }
-    if !first_locked {
-        for f in first {
-            ordered(&[second], *f);
-        }
+    // Each first entity that can move may (a free symmetry line between two fixed lines, #1796),
+    // even when another of them can't.
+    for f in first.iter().filter(|f| !ent_locked(sk, rep, **f)) {
+        ordered(&[second], *f);
     }
     const TEMP: &str = "__keep_size";
     for (h, extra) in tries {

crates/engine/src/cmd/sketch_constraints_tests.rs

@@ -283,3 +283,23 @@ fn an_angle_between_lines_takes_the_reading_nearest_its_value_and_reports_degree
     let d = run(&mut s, "sketch.dimension", json!({"entities": ["l2"], "driven": true}));
     assert_eq!(d["unit"], "mm", "{d}");
 }
+
+#[test]
+fn symmetry_moves_a_free_line_between_two_fixed_lines() {
+    // #1796: with both mirrored lines fixed, the symmetry line between them (a free construction
+    // line) is the one to move; it was refused as "neither entity can move".
+    let mut s = new_sketch();
+    run(&mut s, "sketch.line", json!({"points": [[0, 0], [0, 20]], "ids": ["left"]}));
+    run(&mut s, "sketch.line", json!({"points": [[10, 0], [10, 20]], "ids": ["right"]}));
+    run(&mut s, "sketch.line", json!({"points": [[3, 0], [3, 20]], "construction": true, "ids": ["mid"]}));
+    run(&mut s, "sketch.constraint.fix", json!({"entities": ["left", "right"]}));
+    run(&mut s, "sketch.constraint.symmetry", json!({"a": "left", "b": "right", "line": "mid"}));
+    let sk = sketch(&s);
+    let at = |r: &str| sk.point(sk.resolve_point(r).unwrap()).unwrap();
+    for end in ["mid.start", "mid.end"] {
+        assert!((at(end).x - 5.0).abs() < 1e-7, "{end} at {:?}", at(end));
+    }
+    for (end, x) in [("left.start", 0.0), ("left.end", 0.0), ("right.start", 10.0), ("right.end", 10.0)] {
+        assert!((at(end).x - x).abs() < 1e-9, "{end} moved to {:?}", at(end));
+    }
+}