Preserve canvas context across property edits and contextual drawing

crates/oa-desktop/src/connection_edit.rs

@@ -39,6 +39,24 @@ impl Session {
     }
 }
 
+/// Contextual drawing follows the visible semantic source, not the palette layer.
+/// The caller separately validates the menu/context lifetime and pending drafts.
+pub fn can_begin(project: &Project, diagram: Id, tool: Tool, source: Id) -> bool {
+    let Some(diagram) = project.diagrams.get(&diagram) else {
+        return false;
+    };
+    let Some(element) = project.elements.get(&source) else {
+        return false;
+    };
+    diagram.projection.is_architecture()
+        && (element.layer() == diagram.layer
+            || diagram.projection == oa_core::ProjectionKind::Traceability)
+        && Tool::for_layer(element.layer()).contains(&tool)
+        && tool.can_start(project, source)
+        && (tool != Tool::Allocation || tool.available_in(element.layer(), diagram.projection))
+        && visible(&oa_core::projection::project_view(project, diagram.id)).any(|id| id == source)
+}
+
 pub fn visible(view: &ProjectedView) -> impl Iterator<Item = Id> + '_ {
     view.nodes
         .iter()