A sketch fillet keeps the dimensions of the corner it rounds: filleting the corner of a spout (a vertical riser and a horizontal top, the top's length dimensioned to the corner and the corner's height from the origin) dropped the height dimension with the corner point and left the length on the shortened straight part, so the top came out a fillet radius too long. Like Fusion, a corner that dimensions or constraints measure to now stays as a virtual sharp corner, a free point held on both curves' extensions (two point-on-curve constraints), with those dimensions kept and a line length to the corner turned into a distance from the line's far end to it; chamfers do the same. sketch.fillet and the chamfers report virtual_corners and moved_to_virtual_corner, and the viewport draws a virtual corner as an open square with dashed extension lines from the curves. Tests: the spout keeps its 80 x 100 size and both dimensions still drive it, the virtual corner survives save/load and undo/redo, a fillet with nothing on its corner leaves no point behind, and nearly straight or folded corners with dimensions never panic

crates/engine/src/view.rs

@@ -95,6 +95,13 @@ pub fn sketch_lines(sk: &Sketch, plane: &Plane, active: bool, determined: &[bool
             out.push((seg.polyline(0.02).iter().map(|p| plane.to_world(*p)).collect(), col, c.construction || c.centerline));
         }
     }
+    // Virtual sharp corners (kept by fillets and chamfers): thin extension lines from the
+    // curves to the corner, like construction geometry.
+    for (_, ext) in sk.virtual_corners() {
+        for (a, b) in ext {
+            out.push((vec![plane.to_world(a), plane.to_world(b)], colors::SKETCH_CONSTRUCTION, true));
+        }
+    }
     // 3D curves are in world coordinates already.
     for w in sk.wires.iter().filter(|w| !w.fit.is_empty() || !sk.view.hide_projected) {
         // Curves drawn in a 3D sketch look like the sketch's own; included ones are references.