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/mod.rs

@@ -458,6 +458,7 @@ struct PaperViewportCache {
     layout_block: Handle,
     sheet: Handle,
     content: Arc<Vec<Handle>>,
+    paper_limits: Option<((f64, f64), (f64, f64))>,
 }
 
 struct PaperSheetRenderCache {
@@ -3354,6 +3355,12 @@ impl Scene {
             return None;
         }
 
+        if let Some(cache) = self.paper_viewport_cache.borrow().get(&self.current_layout) {
+            if cache.epoch == self.geometry_epoch && cache.layout == self.current_layout {
+                return cache.paper_limits;
+            }
+        }
+
         self.document
             .objects
             .values()