new_editor: tighten window workspace validation

new_editor/src/Workspace/UIEditorWindowWorkspaceModel.cpp

@@ -1,6 +1,7 @@
 #include <XCEditor/Workspace/UIEditorWindowWorkspaceModel.h>
 #include <XCEditor/Workspace/UIEditorWorkspaceValidation.h>
 
+#include <unordered_map>
 #include <unordered_set>
 #include <utility>
 
@@ -87,6 +88,7 @@ UIEditorWindowWorkspaceValidationResult ValidateUIEditorWindowWorkspaceSet(
     }
 
     std::unordered_set<std::string> seenWindowIds = {};
+    std::unordered_map<std::string, std::string> panelOwners = {};
     bool hasPrimaryWindow = false;
     bool hasActiveWindow = false;
     for (const UIEditorWindowWorkspaceState& state : windowSet.windows) {
@@ -126,6 +128,17 @@ UIEditorWindowWorkspaceValidationResult ValidateUIEditorWindowWorkspaceSet(
                 "Window '" + state.windowId + "' session invalid: " +
                     sessionValidation.message);
         }
+
+        for (const UIEditorPanelSessionState& panelState : state.session.panelStates) {
+            const auto [ownerIt, inserted] =
+                panelOwners.emplace(panelState.panelId, state.windowId);
+            if (!inserted) {
+                return MakeValidationError(
+                    UIEditorWindowWorkspaceValidationCode::DuplicatePanelAcrossWindows,
+                    "Panel '" + panelState.panelId + "' is present in both window '" +
+                        ownerIt->second + "' and window '" + state.windowId + "'.");
+            }
+        }
     }
 
     if (!hasPrimaryWindow) {