deselect build tool when it becomes unaffordable and fix stale enabled button

docs/architecture.md

@@ -97,6 +97,12 @@ All UI interactions — building selection, builder/blueprint mode transitions,
 
 Bidirectional interactions use separate request/notification event types to avoid infinite recursion (e.g., `ExitBuilderModeRequestedEvent` from `BuildButtonGrid` → `GameWorldView`, vs. `BuilderModeExitedEvent` from `GameWorldView` → `BuildButtonGrid`).
 
+### Reading Simulation State
+
+The simulation is the single source of truth for every game value (building block stock, expansion cost, threat level, tick, etc.). A UI widget that needs such a value holds the `Simulation*` it was constructed with and **pulls the value on demand** via the corresponding getter (e.g., `m_sim->getBuildingBlocksStock()`), rather than caching its own copy.
+
+State-change events (e.g., `BuildingBlocksChangedEvent`) are treated as *refresh signals*, not as carriers of truth: a widget subscribes to the event to learn *when* the value changed and then re-reads it from the simulation to learn *what* it now is. The value carried in the event payload is not authoritative and should not be stored. This keeps a single copy of each value and avoids stale-cache bugs (a widget acting on a value that has since moved on because nothing refreshed its local copy).
+
 ## Tick Order
 
 Within a single simulation tick, subsystems run in this fixed order. The order is load-bearing for determinism and for avoiding one-tick-delay artifacts (e.g., items landing on a belt but not advancing in the same tick).

src/ui/BuildButtonGrid.cpp

@@ -12,12 +12,13 @@
 #include "DisplayName.h"
 #include "EventManager.h"
 #include "ExitBuilderModeRequestedEvent.h"
+#include "Simulation.h"
 
 
-BuildButtonGrid::BuildButtonGrid(const GameConfig* config, QWidget* parent)
+BuildButtonGrid::BuildButtonGrid(Simulation* sim, const GameConfig* config, QWidget* parent)
     : QWidget(parent)
+    , m_sim(sim)
     , m_config(config)
-    , m_activeIndex(-1)
 {
     QGridLayout* layout = new QGridLayout(this);
     layout->setSpacing(4);
@@ -79,24 +80,42 @@ BuildButtonGrid::~BuildButtonGrid()
     unregisterForEvents();
 }
 
-void BuildButtonGrid::updateAffordability(int buildingBlocks)
+void BuildButtonGrid::updateAffordability()
 {
+    const int buildingBlocks = m_sim->getBuildingBlocksStock();
+
+    // If the currently selected tool can no longer be afforded, exit builder mode
+    // before recomputing button states so it does not stay selected. Clearing the
+    // active index first lets the loop below disable the now-unaffordable button.
+    if (m_activeIndex)
+    {
+        const BuildingType activeType = m_types[*m_activeIndex];
+        const std::map<BuildingType, int>::const_iterator it = m_costs.find(activeType);
+        const int cost = (it != m_costs.end()) ? it->second : 0;
+        if (buildingBlocks < cost)
+        {
+            clearActiveButton();
+            EventManager::getInstance()->sendEventImmediately(
+                std::make_shared<ExitBuilderModeRequestedEvent>());
+        }
+    }
+
     for (std::size_t i = 0; i < m_buttons.size(); ++i)
     {
         const BuildingType type = m_types[i];
         const std::map<BuildingType, int>::const_iterator it = m_costs.find(type);
         const int cost = (it != m_costs.end()) ? it->second : 0;
-        m_buttons[i]->setEnabled(buildingBlocks >= cost || m_activeIndex == static_cast<int>(i));
+        m_buttons[i]->setEnabled(buildingBlocks >= cost || m_activeIndex == i);
     }
 }
 
 void BuildButtonGrid::clearActiveButton()
 {
-    if (m_activeIndex >= 0 && m_activeIndex < static_cast<int>(m_buttons.size()))
+    if (m_activeIndex)
     {
-        m_buttons[static_cast<std::size_t>(m_activeIndex)]->setChecked(false);
+        m_buttons[*m_activeIndex]->setChecked(false);
     }
-    m_activeIndex = -1;
+    m_activeIndex.reset();
 }
 
 void BuildButtonGrid::onBuildButton(int index)
@@ -105,8 +124,9 @@ void BuildButtonGrid::onBuildButton(int index)
     {
         return;
     }
+    const std::size_t idx = static_cast<std::size_t>(index);
 
-    if (m_activeIndex == index)
+    if (m_activeIndex == idx)
     {
         clearActiveButton();
         EventManager::getInstance()->sendEventImmediately(
@@ -114,15 +134,15 @@ void BuildButtonGrid::onBuildButton(int index)
         return;
     }
 
-    if (m_activeIndex >= 0 && m_activeIndex < static_cast<int>(m_buttons.size()))
+    if (m_activeIndex)
     {
-        m_buttons[static_cast<std::size_t>(m_activeIndex)]->setChecked(false);
+        m_buttons[*m_activeIndex]->setChecked(false);
     }
 
-    m_activeIndex = index;
+    m_activeIndex = idx;
-    m_buttons[static_cast<std::size_t>(index)]->setChecked(true);
+    m_buttons[idx]->setChecked(true);
     EventManager::getInstance()->sendEventImmediately(
-        std::make_shared<BuildingTypeSelectedEvent>(m_types[static_cast<std::size_t>(index)]));
+        std::make_shared<BuildingTypeSelectedEvent>(m_types[idx]));
 }
 
 void BuildButtonGrid::handleEvent(std::shared_ptr<const BuilderModeExitedEvent> /*event*/)
@@ -130,6 +150,11 @@ void BuildButtonGrid::handleEvent(std::shared_ptr<const BuilderModeExitedEvent>
     clearActiveButton();
 }
 
+void BuildButtonGrid::handleEvent(std::shared_ptr<const BuildingBlocksChangedEvent> /*event*/)
+{
+    updateAffordability();
+}
+
 void BuildButtonGrid::handleEvent(std::shared_ptr<const DemolishModeChangedEvent> event)
 {
     m_demolishButton->setChecked(event->active);

src/ui/BuildButtonGrid.h

@@ -1,46 +1,56 @@
 #pragma once
 
 #include <map>
+#include <optional>
 #include <vector>
 
 #include <QWidget>
 
 #include "BuilderModeExitedEvent.h"
 #include "BuildHotkeyPressedEvent.h"
+#include "BuildingBlocksChangedEvent.h"
 #include "BuildingType.h"
 #include "DemolishModeChangedEvent.h"
 #include "EventHandler.h"
 #include "GameConfig.h"
 
 class QPushButton;
+class Simulation;
 
 class BuildButtonGrid : public QWidget,
                         public CombinedEventHandler<BuilderModeExitedEvent,
                                                     DemolishModeChangedEvent,
-                                                    BuildHotkeyPressedEvent>
+                                                    BuildHotkeyPressedEvent,
+                                                    BuildingBlocksChangedEvent>
 {
     Q_OBJECT
 
 public:
-    BuildButtonGrid(const GameConfig* config, QWidget* parent = nullptr);
+    BuildButtonGrid(Simulation* sim, const GameConfig* config, QWidget* parent = nullptr);
     ~BuildButtonGrid() override;
 
-    void updateAffordability(int buildingBlocks);
     void clearActiveButton();
 
 private:
+    // Re-evaluates which build buttons are enabled from the current building block
+    // stock (read from the simulation). If the currently selected tool can no longer
+    // be afforded, it exits builder mode so the button does not stay selected.
+    void updateAffordability();
+
     void handleEvent(std::shared_ptr<const BuilderModeExitedEvent> event) override;
     void handleEvent(std::shared_ptr<const DemolishModeChangedEvent> event) override;
     void handleEvent(std::shared_ptr<const BuildHotkeyPressedEvent> event) override;
+    void handleEvent(std::shared_ptr<const BuildingBlocksChangedEvent> event) override;
 
 private slots:
     void onBuildButton(int index);
 
 private:
+    Simulation*                      m_sim;
     const GameConfig*                m_config;
     std::vector<BuildingType>        m_types;
     std::vector<QPushButton*>        m_buttons;
     std::map<BuildingType, int>      m_costs;
-    int                              m_activeIndex;
+    std::optional<std::size_t>       m_activeIndex;
     QPushButton*                     m_demolishButton;
 };

src/ui/MainWindow.cpp

@@ -14,7 +14,6 @@
 
 #include "BlueprintPanel.h"
 #include "BuildButtonGrid.h"
-#include "BuildingBlocksChangedEvent.h"
 #include "BuildingSystem.h"
 #include "Command.h"
 #include "CommandRequestedEvent.h"
@@ -53,7 +52,7 @@ MainWindow::MainWindow(Simulation* sim, const std::string& configDir,
     sideLayout->setSpacing(1);
 
     m_selectedBuildingPanel = new SelectedBuildingPanel(sim, &sim->getConfig(), m_sidePanel);
-    m_buildButtonGrid       = new BuildButtonGrid(&sim->getConfig(), m_sidePanel);
+    m_buildButtonGrid       = new BuildButtonGrid(sim, &sim->getConfig(), m_sidePanel);
     m_blueprintPanel        = new BlueprintPanel(sim, &sim->getConfig(), m_sidePanel);
 
     sideLayout->addWidget(m_selectedBuildingPanel, 1);
@@ -151,11 +150,6 @@ void MainWindow::layoutPanels()
     m_dimOverlay->setGeometry(0, 0, totalW, totalH);
 }
 
-void MainWindow::handleEvent(std::shared_ptr<const BuildingBlocksChangedEvent> event)
-{
-    m_buildButtonGrid->updateAffordability(event->blocks);
-}
-
 void MainWindow::handleEvent(std::shared_ptr<const SchematicChoicesAvailableEvent> event)
 {
     const double prevSpeed = m_gameWorldView->getGameSpeed();

src/ui/MainWindow.h

@@ -6,7 +6,6 @@
 
 #include <QWidget>
 
-#include "BuildingBlocksChangedEvent.h"
 #include "BuildingId.h"
 #include "EscapeMenuRequestedEvent.h"
 #include "EventHandler.h"
@@ -32,8 +31,7 @@ class QCloseEvent;
 class QResizeEvent;
 
 class MainWindow : public QWidget,
-                   public CombinedEventHandler<BuildingBlocksChangedEvent,
+                   public CombinedEventHandler<SchematicChoicesAvailableEvent,
-                                               SchematicChoicesAvailableEvent,
                                                GameOverEvent,
                                                WinEvent,
                                                EscapeMenuRequestedEvent,
@@ -52,7 +50,6 @@ protected:
     void closeEvent(QCloseEvent* event) override;
 
 private:
-    void handleEvent(std::shared_ptr<const BuildingBlocksChangedEvent> event) override;
     void handleEvent(std::shared_ptr<const SchematicChoicesAvailableEvent> event) override;
     void handleEvent(std::shared_ptr<const GameOverEvent> event) override;
     void handleEvent(std::shared_ptr<const WinEvent> event) override;