Sketch fillet and chamfer: a construction line at the corner is not a side (#1026) (#1176)

Sketch fillet and chamfer: a construction line at the corner is not a side (#1026).

crates/engine/src/cmd/sketch_modify.rs

@@ -488,9 +488,13 @@ fn corner(sk: &Sketch, p: &Value, cmd: &str) -> Result<(usize, usize, usize, usi
     let (la, lb) = if let Some(r) = str_(p, "point") {
         let q = sk.resolve_point(r).ok_or_else(|| bad(cmd, format!("unknown point `{r}`")))?;
         let lines: Vec<usize> = (0..sk.curves.len()).filter(|i| ends(*i).is_some_and(|(a, b)| a == q || b == q) && !sk.is_linked_curve(*i)).collect();
-        match lines[..] {
+        // A construction line ending there too (a diagonal that centres something) is not one of
+        // the corner's sides: it keeps its end on the virtual sharp corner (#1026).
+        let sides: Vec<usize> =
+            if lines.len() == 2 { lines } else { lines.into_iter().filter(|i| sk.curves.get(*i).is_some_and(|c| !c.construction)).collect() };
+        match sides[..] {
             [a, b] => (a, b),
-            _ => return Err(bad(cmd, "the corner must join exactly two lines")),
+            _ => return Err(bad(cmd, "the corner must join exactly two lines (or give the two lines as `a` and `b`)")),
         }
     } else {
         let a = str_(p, "a").and_then(|i| sk.curve_index(i)).ok_or_else(|| bad(cmd, "`a` must be a line"))?;