fix bug where selecting the same layout for a shipyard discarded the current progress and buffers

src/lib/sim/BuildingSystem.cpp

@@ -284,6 +284,21 @@ void BuildingSystem::setShipLayout(FactoryState& state, BuildingId id, const Shi
     {
         if (building.id == id)
         {
+            // No-op if the layout is unchanged, so re-applying the layout a shipyard
+            // already has does not cancel its production cycle or wipe its buffers
+            // (REQ-MAT-INPUT-BUFFER, REQ-BLD-SHIPYARD). Confirming the layout dialog
+            // without editing anything, and a blueprint configuration transfer onto an
+            // already-matching shipyard (REQ-UI-BLUEPRINT-TRANSFER), both land here.
+            // An unset layout counts as an empty one: the two are equivalent for
+            // buffers, production, and the spawned ship (see the spawn path below),
+            // so an empty layout arriving at an unconfigured shipyard changes nothing.
+            const bool unchanged = building.shipLayout.has_value()
+                                       ? *building.shipLayout == layout
+                                       : layout.placedModules.empty();
+            if (unchanged)
+            {
+                return;
+            }
             if (building.production.has_value())
             {
                 building.production = std::nullopt;

src/lib/sim/ShipLayout.h

@@ -15,8 +15,23 @@ struct PlacedModule
     Rotation rotation;
 };
 
+inline bool operator==(const PlacedModule& a, const PlacedModule& b)
+{
+    return a.moduleId == b.moduleId && a.position == b.position && a.rotation == b.rotation;
+}
+
 // The complete module configuration for a shipyard's current ship (REQ-MOD-CONFIG).
 struct ShipLayoutConfig
 {
     std::vector<PlacedModule> placedModules;
 };
+
+// Deliberately order-sensitive: two layouts holding the same modules in a different
+// vector order compare unequal. This only decides whether applying a layout is a no-op
+// (REQ-MAT-INPUT-BUFFER), so the conservative answer is the safe one -- reporting a
+// change that is not one costs a buffer reset, reporting no change when there is one
+// would leave the shipyard stale.
+inline bool operator==(const ShipLayoutConfig& a, const ShipLayoutConfig& b)
+{
+    return a.placedModules == b.placedModules;
+}