fix(viewport): adapt layout crosshair contrast

Choose the sheet or desk surface under the cursor instead of the active input space so MSPACE stays visible on light paper.\n\nRefs #611

src/scene/limits.rs

@@ -118,6 +118,9 @@ impl Scene {
                 }
             }
         }
+        self.paper_viewport_cache
+            .borrow_mut()
+            .remove(&self.current_layout);
     }
 
     /// ZOOM All frames the configured drawing limits. Object-only framing