fix(grips): a grip lands on the snap marker it shows

While a grip drags, the edited entity is hidden from snapping and its
drag-start shape got a reference-only pass: the marker appeared on the
old edge's midpoint or corner, but the grip stayed at the raw cursor,
and the pass ran only with object snap tracking on. The reference pass
now always runs, still limited to discrete points (never nearest-style,
so the point cannot be pulled back onto the old shape) and skipping the
grip's own start point, and when no snap on the rest of the drawing wins
it places the grip too.

Fixes #1528

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/app/update/viewport.rs

@@ -1439,13 +1439,14 @@ impl OpenCADStudio {
                 construction_ray,
             );
 
-            // The frozen pre-drag geometry gets a SECOND, reference-only snap pass.
-            //
-            // Its result is never used to place/move the grip. It exists only so the
-            // user can hover an endpoint/midpoint/etc. of the original geometry and
-            // acquire it for OTRACK exactly like ordinary drawing geometry.
+            // The frozen pre-drag geometry gets a SECOND snap pass, limited to
+            // its discrete points: a corner can be dropped on the old midpoint
+            // of its own edge, or acquired for OTRACK, while nearest/tangent-
+            // style snaps can never pull the point back onto the old shape.
+            // The grip's own starting point is skipped so it does not stick
+            // where the drag began.
             let reference_snap_hit =
-                if self.snapper.tracking_active() && !self.grip_reference_wires.is_empty() {
+                if !self.grip_reference_wires.is_empty() {
                     self.snapper
                         .snap(
                             raw,
@@ -1469,21 +1470,21 @@ impl OpenCADStudio {
                                     | crate::snap::SnapType::Intersection
                                     | crate::snap::SnapType::Insertion
                                     | crate::snap::SnapType::ApparentIntersection
-                            )
+                            ) && hit.world.distance(grip.origin_world) > 1e-9
                         })
                 } else {
                     None
                 };
 
-            // The visible marker should normally describe the real snap that is driving
-            // the cursor. However, when there is no real object snap (or only Grid), show
-            // the reference snap marker so the user can see what point is being acquired.
-            let display_snap_hit = match snap_hit {
+            // A real object snap on the rest of the drawing wins; otherwise the
+            // reference point both shows and places the grip, so the marker is
+            // always the point the grip lands on.
+            let snap_hit = match snap_hit {
                 Some(hit) if hit.snap_type != crate::snap::SnapType::Grid => Some(hit),
                 _ => reference_snap_hit.or(snap_hit),
             };
 
-            self.tabs[i].snap_result = display_snap_hit;
+            self.tabs[i].snap_result = snap_hit;
 
             // OTRACK acquisition needs access to the original wire geometry so, once a
             // reference point has dwelt long enough, it can capture the segment directions