move the field selection out of SelectedBuildingPanel

The two selection categories used to arbitrate ownership of the panel by
poking each other's widgets: buildFieldSelection() called clearContent()
and buildEmpty(), buildEmpty() hid the four entity widgets, and
hideAllWidgets() hid the scrap label. Splitting the halves apart without
naming an arbiter would only have spread that across a class boundary.

SelectedBuildingPanel is now the sole arbiter. It still receives all
three selection events, forwards the two field ones to the embedded
FieldSelectionPanel, and drops its own selection and content as soon as
the field panel reports a selection (yieldToFieldSelection), mirroring
what onSelectionChanged() already did in the other direction. The field
panel decides only what to render and whether it is visible at all.

Dropping the field branch of refreshSelectionDisplay() is behaviour
preserving: whenever the field category owns the panel, m_singleBuildingId
is null, so the building refresh returns immediately anyway.

clearContent() and buildEmpty() became identical once the cross-half
hiding was gone, so only buildEmpty() remains.

Co-Authored-By: Claude <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YHcUerKAZKWNvSKJxYKbnG

src/ui/CMakeLists.txt

@@ -9,6 +9,7 @@ SET(HDRS
     ${CMAKE_CURRENT_SOURCE_DIR}/HeaderBar.h
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildButtonGrid.h
     ${CMAKE_CURRENT_SOURCE_DIR}/SelectedBuildingPanel.h
+    ${CMAKE_CURRENT_SOURCE_DIR}/FieldSelectionPanel.h
     ${CMAKE_CURRENT_SOURCE_DIR}/BlueprintPanel.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutDialog.h
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutPreview.h
@@ -30,6 +31,7 @@ SET(SRCS
     ${CMAKE_CURRENT_SOURCE_DIR}/HeaderBar.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BuildButtonGrid.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/SelectedBuildingPanel.cpp
+    ${CMAKE_CURRENT_SOURCE_DIR}/FieldSelectionPanel.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/BlueprintPanel.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutDialog.cpp
     ${CMAKE_CURRENT_SOURCE_DIR}/ShipLayoutPreview.cpp

src/ui/FieldSelectionPanel.cpp

@@ -0,0 +1,403 @@
+#include "FieldSelectionPanel.h"
+
+#include <algorithm>
+#include <map>
+#include <string>
+
+#include <QFont>
+#include <QLabel>
+#include <QStringList>
+#include <QVBoxLayout>
+
+#include "DebrisSystem.h"
+#include "DisplayName.h"
+#include "EntityAdmin.h"
+#include "FactionComponent.h"
+#include "GameConfig.h"
+#include "HealthComponent.h"
+#include "ModuleOwnerComponent.h"
+#include "SelectedBehaviorComponent.h"
+#include "ShipIdentityComponent.h"
+#include "ShipStatsCalculator.h"
+#include "ShipStatsPanel.h"
+#include "Simulation.h"
+#include "StationBodyComponent.h"
+#include "ThreatCostCalculator.h"
+#include "WeaponComponent.h"
+
+
+FieldSelectionPanel::FieldSelectionPanel(Simulation* sim,
+                                         const GameConfig* config,
+                                         QWidget* parent)
+    : QWidget(parent)
+    , m_sim(sim)
+    , m_config(config)
+{
+    // Zero margins and the same spacing as the enclosing SelectedBuildingPanel layout, so
+    // nesting the field widgets in this panel leaves their geometry unchanged.
+    m_layout = new QVBoxLayout(this);
+    m_layout->setContentsMargins(0, 0, 0, 0);
+    m_layout->setSpacing(4);
+    m_layout->setAlignment(Qt::AlignTop);
+
+    m_entityTitleLabel = new QLabel(this);
+    QFont titleFont = m_entityTitleLabel->font();
+    titleFont.setBold(true);
+    m_entityTitleLabel->setFont(titleFont);
+    m_layout->addWidget(m_entityTitleLabel);
+    m_entityTitleLabel->hide();
+
+    m_entityStatsPanel = new ShipStatsPanel(config, this);
+    m_layout->addWidget(m_entityStatsPanel);
+    m_entityStatsPanel->hide();
+
+    m_stationStatsLabel = new QLabel(this);
+    m_stationStatsLabel->setWordWrap(true);
+    m_layout->addWidget(m_stationStatsLabel);
+    m_stationStatsLabel->hide();
+
+    m_entitySummaryLabel = new QLabel(this);
+    m_entitySummaryLabel->setWordWrap(true);
+    m_layout->addWidget(m_entitySummaryLabel);
+    m_entitySummaryLabel->hide();
+
+    m_scrapLabel = new QLabel(this);
+    m_layout->addWidget(m_scrapLabel);
+    m_scrapLabel->hide();
+
+    hide();
+
+    registerForEvents();
+}
+
+FieldSelectionPanel::~FieldSelectionPanel()
+{
+    unregisterForEvents();
+}
+
+void FieldSelectionPanel::setSelectedEntities(const std::vector<entt::entity>& entities)
+{
+    m_selectedEntities = entities;
+    rebuild();
+}
+
+void FieldSelectionPanel::setSelectedDebris(const std::vector<entt::entity>& debris)
+{
+    m_selectedDebris = debris;
+    rebuild();
+}
+
+void FieldSelectionPanel::clearSelection()
+{
+    m_selectedEntities.clear();
+    m_selectedDebris.clear();
+    rebuild();
+}
+
+bool FieldSelectionPanel::hasSelection() const
+{
+    return !m_selectedEntities.empty() || !m_selectedDebris.empty();
+}
+
+void FieldSelectionPanel::hideAllWidgets()
+{
+    m_entityTitleLabel->hide();
+    m_entityStatsPanel->hide();
+    m_stationStatsLabel->hide();
+    m_entitySummaryLabel->hide();
+    m_scrapLabel->hide();
+}
+
+void FieldSelectionPanel::rebuild()
+{
+    if (!hasSelection())
+    {
+        // Nothing in the field category: take no space, leaving the panel to whatever
+        // the building category shows (REQ-UI-SELECTION-CATEGORIES).
+        hideAllWidgets();
+        hide();
+        return;
+    }
+
+    show();
+
+    EntityAdmin& admin = m_sim->getAdmin();
+
+    // A full single-object stats panel is shown only for a lone field object: one actor
+    // with no debris, or one piece of debris with no actors. As soon as the selection holds
+    // more than one object (multiple actors, multiple debris, or actors plus debris), the
+    // panel switches to the compact count summary (REQ-UI-FIELD-MULTI-SELECTION).
+    if (m_selectedEntities.size() == 1 && m_selectedDebris.empty())
+    {
+        m_entitySummaryLabel->hide();
+        m_scrapLabel->hide();
+        const entt::entity entity = m_selectedEntities.front();
+        if (admin.isValid(entity) && admin.hasAll<ShipIdentityComponent>(entity))
+        {
+            buildEntityShip(entity);
+        }
+        else if (admin.isValid(entity) && admin.hasAll<StationBodyComponent>(entity))
+        {
+            buildEntityStation(entity);
+        }
+        else
+        {
+            m_entityTitleLabel->hide();
+            m_entityStatsPanel->hide();
+            m_stationStatsLabel->hide();
+        }
+        return;
+    }
+
+    if (m_selectedEntities.empty() && m_selectedDebris.size() == 1)
+    {
+        // Single piece of debris: a "Debris" heading plus a "Scrap" stat row, styled like
+        // the ship/station stats panels (REQ-UI-DEBRIS-PANEL).
+        m_entitySummaryLabel->hide();
+        m_entityStatsPanel->hide();
+        m_stationStatsLabel->hide();
+        buildDebrisSingle();
+        return;
+    }
+
+    // More than one field object: a compact count summary. buildEntitySummary() appends the
+    // "Debris x N" and "Scrap x N" lines when debris is part of the selection.
+    m_entityTitleLabel->hide();
+    m_entityStatsPanel->hide();
+    m_stationStatsLabel->hide();
+    m_scrapLabel->hide();
+    buildEntitySummary();
+}
+
+void FieldSelectionPanel::refreshDisplay()
+{
+    if (!hasSelection()) { return; }
+
+    // Keep the live values current: the single-actor stats panel, the single-debris stats
+    // panel (whose Scrap row shrinks as it is collected), or the count summary (whose Scrap
+    // line shrinks likewise) — matching the layout chosen by rebuild()
+    // (REQ-UI-SHIP-STATS-PANEL, REQ-UI-DEBRIS-PANEL).
+    if (m_selectedEntities.size() == 1 && m_selectedDebris.empty())
+    {
+        refreshEntityStats();
+    }
+    else if (m_selectedEntities.empty() && m_selectedDebris.size() == 1)
+    {
+        buildDebrisSingle();
+    }
+    else
+    {
+        buildEntitySummary();
+    }
+}
+
+void FieldSelectionPanel::buildDebrisSingle()
+{
+    // "Debris" heading + a single "Scrap" stat row for the piece's remaining amount,
+    // mirroring the single-actor stats panels (REQ-UI-DEBRIS-PANEL).
+    m_entityTitleLabel->setText(tr("Debris"));
+    m_entityTitleLabel->show();
+    m_scrapLabel->setText(tr("Scrap: %1").arg(selectedDebrisScrapTotal()));
+    m_scrapLabel->show();
+}
+
+void FieldSelectionPanel::buildEntitySummary()
+{
+    EntityAdmin& admin = m_sim->getAdmin();
+
+    // Group actors by faction + kind + ship schematic, preserving first-seen order
+    // (REQ-UI-FIELD-MULTI-SELECTION).
+    std::vector<QString> keys;
+    std::map<QString, int> counts;
+    std::map<QString, QString> labels;
+
+    for (entt::entity entity : m_selectedEntities)
+    {
+        if (!admin.isValid(entity)) { continue; }
+        const bool isEnemy = admin.hasAll<FactionComponent>(entity)
+            && admin.get<FactionComponent>(entity).isEnemy;
+
+        QString key;
+        QString label;
+        if (admin.hasAll<ShipIdentityComponent>(entity))
+        {
+            const std::string& id = admin.get<ShipIdentityComponent>(entity).schematicId;
+            const QString name = QString::fromStdString(toDisplayName(id));
+            key = (isEnemy ? QStringLiteral("ship:enemy:") : QStringLiteral("ship:player:"))
+                + QString::fromStdString(id);
+            label = isEnemy ? tr("Enemy %1").arg(name) : name;
+        }
+        else if (admin.hasAll<StationBodyComponent>(entity))
+        {
+            key = isEnemy ? QStringLiteral("station:enemy") : QStringLiteral("station:player");
+            label = isEnemy ? tr("Enemy Defence Station") : tr("Player Defence Station");
+        }
+        else
+        {
+            continue;
+        }
+
+        if (counts.find(key) == counts.end())
+        {
+            keys.push_back(key);
+            labels[key] = label;
+        }
+        counts[key] += 1;
+    }
+
+    // One "<type> x <count>" line per group (matching the recipe tooltip and the building
+    // multi-selection). No total-count header, consistent with the building panel. When
+    // debris is part of the selection, a "Debris x <count>" line followed by a
+    // "Scrap x <total>" line are appended into the same label so the line spacing is
+    // uniform (REQ-UI-FIELD-MULTI-SELECTION, REQ-UI-DEBRIS-PANEL).
+    QStringList lines;
+    for (const QString& key : keys)
+    {
+        lines << tr("%1 x %2").arg(labels[key]).arg(counts[key]);
+    }
+    if (!m_selectedDebris.empty())
+    {
+        lines << tr("Debris x %1").arg(static_cast<int>(m_selectedDebris.size()));
+        lines << scrapTotalText();
+    }
+    m_entitySummaryLabel->setText(lines.join('\n'));
+    m_entitySummaryLabel->show();
+}
+
+void FieldSelectionPanel::buildEntityShip(entt::entity entity)
+{
+    EntityAdmin& admin = m_sim->getAdmin();
+    const ShipIdentityComponent& identity = admin.get<ShipIdentityComponent>(entity);
+    const HealthComponent& health = admin.get<HealthComponent>(entity);
+
+    m_entityTitleLabel->setText(tr("Ship: %1")
+        .arg(QString::fromStdString(identity.schematicId)));
+    m_entityTitleLabel->show();
+
+    const ShipStats stats = buildShipStatsFromEntity(admin, entity);
+    m_entityStatsPanel->refreshFromLive(stats, health.hp);
+    m_entityStatsPanel->setBehavior(
+        admin.get<SelectedBehaviorComponent>(entity).winner);
+    m_entityStatsPanel->setDebugDrawEnabled(m_debugDraw);
+
+    const ShipDef* schematicDef =
+        m_config->ships.findShipDef(identity.schematicId);
+    if (schematicDef)
+    {
+        const double threat = calculateShipThreatCost(
+            m_config->threatCosts, *m_config, schematicDef->id,
+            schematicDef->defaultModules);
+        m_entityStatsPanel->setThreatCost(threat);
+    }
+
+    m_entityStatsPanel->show();
+
+    m_stationStatsLabel->hide();
+}
+
+void FieldSelectionPanel::buildEntityStation(entt::entity entity)
+{
+    EntityAdmin& admin = m_sim->getAdmin();
+    const HealthComponent& health = admin.get<HealthComponent>(entity);
+
+    const bool isEnemy = admin.hasAll<FactionComponent>(entity)
+        && admin.get<FactionComponent>(entity).isEnemy;
+    m_entityTitleLabel->setText(isEnemy
+        ? tr("Enemy Defence Station")
+        : tr("Player Defence Station"));
+    m_entityTitleLabel->show();
+
+    float totalDps = 0.0f;
+    float maxRange = 0.0f;
+    bool hasWeapons = false;
+
+    admin.forEach<ModuleOwnerComponent, WeaponComponent>(
+        [&](entt::entity /*child*/, const ModuleOwnerComponent& owner, const WeaponComponent& w)
+        {
+            if (owner.owner != entity) { return; }
+            hasWeapons = true;
+            totalDps += w.damage * w.fireRateHz;
+            if (w.range_tiles > maxRange) { maxRange = w.range_tiles; }
+        });
+
+    QString statsText = tr("HP: %1 / %2")
+        .arg(static_cast<int>(health.hp + 0.5f))
+        .arg(static_cast<int>(health.maxHp + 0.5f));
+
+    if (hasWeapons)
+    {
+        statsText += tr("\nDPS: %1").arg(QString::number(static_cast<double>(totalDps), 'f', 1));
+        statsText += tr("\nRange: %1 tiles").arg(QString::number(static_cast<double>(maxRange), 'f', 1));
+    }
+
+    m_stationStatsLabel->setText(statsText);
+    m_stationStatsLabel->show();
+
+    m_entityStatsPanel->hide();
+}
+
+void FieldSelectionPanel::refreshEntityStats()
+{
+    // Only the single-actor stats panel needs a live refresh; the multi-actor summary is
+    // static counts, and GameWorldView prunes dead/despawned actors and re-emits the
+    // selection (REQ-UI-ENTITY-CLICK-SELECT), so the panel does not mutate it here.
+    if (m_selectedEntities.size() != 1) { return; }
+
+    EntityAdmin& admin = m_sim->getAdmin();
+    const entt::entity entity = m_selectedEntities.front();
+
+    if (!admin.isValid(entity) || !admin.hasAll<HealthComponent>(entity)) { return; }
+    const HealthComponent& health = admin.get<HealthComponent>(entity);
+    if (health.hp <= 0.0f) { return; }
+
+    if (admin.hasAll<ShipIdentityComponent>(entity))
+    {
+        const ShipStats stats = buildShipStatsFromEntity(admin, entity);
+        m_entityStatsPanel->refreshFromLive(stats, health.hp);
+        m_entityStatsPanel->setBehavior(
+            admin.get<SelectedBehaviorComponent>(entity).winner);
+    }
+    else if (admin.hasAll<StationBodyComponent>(entity))
+    {
+        buildEntityStation(entity);
+    }
+}
+
+int FieldSelectionPanel::selectedDebrisScrapTotal() const
+{
+    // Sum the remaining scrap across the still-living selected debris (REQ-UI-DEBRIS-PANEL).
+    int total = 0;
+    for (const DebrisInfo& info : m_sim->getDebrisSystem().getAllDebrisInfo())
+    {
+        if (std::find(m_selectedDebris.begin(), m_selectedDebris.end(), info.entity)
+            != m_selectedDebris.end())
+        {
+            total += info.amount;
+        }
+    }
+    return total;
+}
+
+QString FieldSelectionPanel::scrapTotalText() const
+{
+    return tr("Scrap x %1").arg(selectedDebrisScrapTotal());
+}
+
+void FieldSelectionPanel::handleEvent(std::shared_ptr<const TickAdvancedEvent> /*event*/)
+{
+    refreshDisplay();
+}
+
+void FieldSelectionPanel::handleEvent(
+    std::shared_ptr<const PlayerCommandsAppliedEvent> /*event*/)
+{
+    // Player commands are applied by a queued drain, not synchronously. When the game is
+    // paused no tick advances, so TickAdvancedEvent never fires; refresh here too.
+    refreshDisplay();
+}
+
+void FieldSelectionPanel::handleEvent(std::shared_ptr<const DebugDrawToggledEvent> event)
+{
+    m_debugDraw = event->active;
+    m_entityStatsPanel->setDebugDrawEnabled(event->active);
+}

src/ui/FieldSelectionPanel.h

@@ -0,0 +1,91 @@
+#pragma once
+
+#include <vector>
+
+#include <QString>
+#include <QWidget>
+
+#include "entt/entity/entity.hpp"
+
+#include "DebugDrawToggledEvent.h"
+#include "EventHandler.h"
+#include "PlayerCommandsAppliedEvent.h"
+#include "TickAdvancedEvent.h"
+
+struct GameConfig;
+class Simulation;
+class ShipStatsPanel;
+class QLabel;
+class QVBoxLayout;
+
+// Renders the "field" selection category — ships, defence stations and debris — as either
+// a single-object stats panel (ship, station, or debris) or a compact multi-object count
+// summary (REQ-UI-SELECTION-CATEGORIES, REQ-UI-FIELD-MULTI-SELECTION, REQ-UI-DEBRIS-PANEL).
+//
+// The panel owns its own selection state and its own widgets, and nothing else. Which of
+// the two selection categories owns the side panel is arbitrated by the parent
+// SelectedBuildingPanel: it feeds this panel through setSelectedEntities() /
+// setSelectedDebris() / clearSelection() and asks it via hasSelection(). This panel hides
+// itself whenever its selection is empty, so an inactive field category takes no space.
+class FieldSelectionPanel : public QWidget,
+                            public CombinedEventHandler<TickAdvancedEvent,
+                                                        PlayerCommandsAppliedEvent,
+                                                        DebugDrawToggledEvent>
+{
+    Q_OBJECT
+
+public:
+    FieldSelectionPanel(Simulation* sim, const GameConfig* config,
+                        QWidget* parent = nullptr);
+    ~FieldSelectionPanel() override;
+
+    // Replaces the selected actors (ships and defence stations); debris is left alone,
+    // the two coexist within the field category (REQ-UI-SELECTION-CATEGORIES).
+    void setSelectedEntities(const std::vector<entt::entity>& entities);
+    // Replaces the selected debris; the selected actors are left alone.
+    void setSelectedDebris(const std::vector<entt::entity>& debris);
+    // Drops the whole field selection — used when the building category takes over.
+    void clearSelection();
+    // True while the field category has anything selected, i.e. while this panel owns
+    // the side panel's content.
+    bool hasSelection() const;
+
+private:
+    void handleEvent(std::shared_ptr<const TickAdvancedEvent> event) override;
+    void handleEvent(std::shared_ptr<const PlayerCommandsAppliedEvent> event) override;
+    void handleEvent(std::shared_ptr<const DebugDrawToggledEvent> event) override;
+
+    // Picks the layout for the current selection and shows/hides this panel accordingly.
+    void rebuild();
+    // Keeps the live values of the layout chosen by rebuild() current.
+    void refreshDisplay();
+    void buildEntityShip(entt::entity entity);
+    void buildEntityStation(entt::entity entity);
+    void buildEntitySummary();
+    void buildDebrisSingle();
+    void refreshEntityStats();
+    void hideAllWidgets();
+    // Summed remaining scrap across the selected debris (REQ-UI-DEBRIS-PANEL).
+    int selectedDebrisScrapTotal() const;
+    // "Scrap x N" line for the multi-object summary (REQ-UI-FIELD-MULTI-SELECTION).
+    QString scrapTotalText() const;
+
+    Simulation*       m_sim;
+    const GameConfig* m_config;
+
+    bool m_debugDraw = false;
+
+    // The selected ships/defence stations. Shares the "field" selection category with
+    // debris (m_selectedDebris): both can be non-empty at once (REQ-UI-SELECTION-CATEGORIES).
+    std::vector<entt::entity> m_selectedEntities;
+    std::vector<entt::entity> m_selectedDebris;
+
+    QVBoxLayout*    m_layout;
+    QLabel*         m_entityTitleLabel;
+    ShipStatsPanel* m_entityStatsPanel;
+    QLabel*         m_stationStatsLabel;
+    QLabel*         m_entitySummaryLabel;
+    // Shows the debris "Scrap" stat row (single selection) — the scrap total for the
+    // multi-object summary lives in m_entitySummaryLabel instead.
+    QLabel*         m_scrapLabel;
+};

src/ui/SelectedBuildingPanel.cpp

@@ -9,26 +9,14 @@
 #include <QLabel>
 #include <QListWidget>
 #include <QPushButton>
-#include <QStringList>
 #include <QVBoxLayout>
 
 #include "BeltSystem.h"
 #include "Command.h"
 #include "CommandRequestedEvent.h"
-#include "DisplayName.h"
-#include "DynamicBodyComponent.h"
-#include "EntityAdmin.h"
 #include "EntitySelectionChangedEvent.h"
 #include "EventManager.h"
-#include "FactionComponent.h"
-#include "HealthComponent.h"
-#include "ModuleOwnerComponent.h"
-#include "SelectedBehaviorComponent.h"
-#include "ShipIdentityComponent.h"
-#include "ShipStatsCalculator.h"
-#include "ShipStatsPanel.h"
-#include "ThreatCostCalculator.h"
-#include "StationBodyComponent.h"
+#include "FieldSelectionPanel.h"
 #include "TickAdvancedEvent.h"
 #include "Building.h"
 #include "BuildingSystem.h"
@@ -40,10 +28,8 @@
 #include "RecipeSelectionDialog.h"
 #include "RecipeSelectionRequestedEvent.h"
 #include "Rotation.h"
-#include "DebrisSystem.h"
 #include "ShipLayoutPreview.h"
 #include "Simulation.h"
-#include "WeaponComponent.h"
 
 namespace
 {
@@ -182,30 +168,10 @@ SelectedBuildingPanel::SelectedBuildingPanel(Simulation* sim,
     connect(m_filterBList, &QListWidget::itemChanged,
             this, &SelectedBuildingPanel::onSplitterFilterChanged);
 
-    m_entityTitleLabel = new QLabel(this);
-    QFont titleFont = m_entityTitleLabel->font();
-    titleFont.setBold(true);
-    m_entityTitleLabel->setFont(titleFont);
-    m_layout->addWidget(m_entityTitleLabel);
-    m_entityTitleLabel->hide();
-
-    m_entityStatsPanel = new ShipStatsPanel(config, this);
-    m_layout->addWidget(m_entityStatsPanel);
-    m_entityStatsPanel->hide();
-
-    m_stationStatsLabel = new QLabel(this);
-    m_stationStatsLabel->setWordWrap(true);
-    m_layout->addWidget(m_stationStatsLabel);
-    m_stationStatsLabel->hide();
-
-    m_entitySummaryLabel = new QLabel(this);
-    m_entitySummaryLabel->setWordWrap(true);
-    m_layout->addWidget(m_entitySummaryLabel);
-    m_entitySummaryLabel->hide();
-
-    m_scrapLabel = new QLabel(this);
-    m_layout->addWidget(m_scrapLabel);
-    m_scrapLabel->hide();
+    // The field selection renders below the building content and hides itself while
+    // nothing field-side is selected, so it costs no space then.
+    m_fieldSelectionPanel = new FieldSelectionPanel(sim, config, this);
+    m_layout->addWidget(m_fieldSelectionPanel);
 
     buildEmpty();
 
@@ -224,13 +190,21 @@ void SelectedBuildingPanel::onSelectionChanged(const std::vector<BuildingId>& id
     {
         // A building selection is exclusive: it supersedes any field selection —
         // actors and scrap (REQ-UI-SELECTION-CATEGORIES).
-        clearEntityDisplay();
-        m_selectedDebris.clear();
-        m_scrapLabel->hide();
+        m_fieldSelectionPanel->clearSelection();
     }
     rebuild();
 }
 
+void SelectedBuildingPanel::yieldToFieldSelection()
+{
+    // The mirror image of onSelectionChanged(): a field selection — actors, debris, or
+    // both — supersedes any building selection (REQ-UI-SELECTION-CATEGORIES). An empty
+    // field selection changes nothing here: the building content, if any, keeps the panel.
+    if (!m_fieldSelectionPanel->hasSelection()) { return; }
+    m_selectedBuildingIds.clear();
+    buildEmpty();
+}
+
 void SelectedBuildingPanel::rebuild()
 {
     if (m_selectedBuildingIds.empty())
@@ -259,22 +233,14 @@ void SelectedBuildingPanel::hideAllWidgets()
     m_filterBLabel->hide();
     m_filterBList->hide();
     m_buffersLabel->hide();
-    m_scrapLabel->hide();
-}
-
-void SelectedBuildingPanel::clearContent()
-{
-    m_singleBuildingId = std::nullopt;
-    hideAllWidgets();
 }
 
 void SelectedBuildingPanel::buildEmpty()
 {
-    clearContent();
-    m_entityTitleLabel->hide();
-    m_entityStatsPanel->hide();
-    m_stationStatsLabel->hide();
-    m_entitySummaryLabel->hide();
+    // Shows nothing for the building category — either because nothing is selected or
+    // because the field category has taken the panel over.
+    m_singleBuildingId = std::nullopt;
+    hideAllWidgets();
 }
 
 void SelectedBuildingPanel::buildSingle(BuildingId id)
@@ -646,27 +612,9 @@ void SelectedBuildingPanel::handleEvent(
 
 void SelectedBuildingPanel::refreshSelectionDisplay(RefreshReason reason)
 {
-    if (!m_selectedEntities.empty() || !m_selectedDebris.empty())
-    {
-        // Field selection. Keep the live values current: the single-actor stats panel,
-        // the single-debris stats panel (whose Scrap row shrinks as it is collected), or
-        // the count summary (whose Scrap line shrinks likewise) — matching the layout
-        // chosen by buildFieldSelection() (REQ-UI-SHIP-STATS-PANEL, REQ-UI-DEBRIS-PANEL).
-        if (m_selectedEntities.size() == 1 && m_selectedDebris.empty())
-        {
-            refreshEntityStats();
-        }
-        else if (m_selectedEntities.empty() && m_selectedDebris.size() == 1)
-        {
-            buildDebrisSingle();
-        }
-        else
-        {
-            buildEntitySummary();
-        }
-        return;
-    }
-
+    // Only a single selected building has live content to refresh. While the field
+    // category owns the panel there is none: yieldToFieldSelection() has cleared it, so
+    // this returns immediately and the field panel refreshes itself off the same events.
     if (!m_singleBuildingId.has_value()) { return; }
     const Building* b = m_sim->getBuildings().findBuilding(*m_singleBuildingId);
     if (b)
@@ -904,259 +852,8 @@ void SelectedBuildingPanel::onClearBelt()
 
 void SelectedBuildingPanel::handleEvent(std::shared_ptr<const EntitySelectionChangedEvent> event)
 {
-    m_selectedEntities = event->entities;
-    if (!m_selectedEntities.empty())
-    {
-        // A field selection supersedes any building selection (REQ-UI-SELECTION-CATEGORIES).
-        m_selectedBuildingIds.clear();
-    }
-    buildFieldSelection();
-}
-
-void SelectedBuildingPanel::buildFieldSelection()
-{
-    if (m_selectedEntities.empty() && m_selectedDebris.empty())
-    {
-        // Nothing in the field category. Fall back to empty unless buildings own the panel.
-        clearEntityDisplay();
-        m_scrapLabel->hide();
-        if (m_selectedBuildingIds.empty())
-        {
-            buildEmpty();
-        }
-        return;
-    }
-
-    // A field selection owns the panel: drop any building content.
-    clearContent();
-
-    EntityAdmin& admin = m_sim->getAdmin();
-
-    // A full single-object stats panel is shown only for a lone field object: one actor
-    // with no debris, or one piece of debris with no actors. As soon as the selection holds
-    // more than one object (multiple actors, multiple debris, or actors plus debris), the
-    // panel switches to the compact count summary (REQ-UI-FIELD-MULTI-SELECTION).
-    if (m_selectedEntities.size() == 1 && m_selectedDebris.empty())
-    {
-        m_entitySummaryLabel->hide();
-        m_scrapLabel->hide();
-        const entt::entity entity = m_selectedEntities.front();
-        if (admin.isValid(entity) && admin.hasAll<ShipIdentityComponent>(entity))
-        {
-            buildEntityShip(entity);
-        }
-        else if (admin.isValid(entity) && admin.hasAll<StationBodyComponent>(entity))
-        {
-            buildEntityStation(entity);
-        }
-        else
-        {
-            m_entityTitleLabel->hide();
-            m_entityStatsPanel->hide();
-            m_stationStatsLabel->hide();
-        }
-        return;
-    }
-
-    if (m_selectedEntities.empty() && m_selectedDebris.size() == 1)
-    {
-        // Single piece of debris: a "Debris" heading plus a "Scrap" stat row, styled like
-        // the ship/station stats panels (REQ-UI-DEBRIS-PANEL).
-        m_entitySummaryLabel->hide();
-        m_entityStatsPanel->hide();
-        m_stationStatsLabel->hide();
-        buildDebrisSingle();
-        return;
-    }
-
-    // More than one field object: a compact count summary. buildEntitySummary() appends the
-    // "Debris x N" and "Scrap x N" lines when debris is part of the selection.
-    m_entityTitleLabel->hide();
-    m_entityStatsPanel->hide();
-    m_stationStatsLabel->hide();
-    m_scrapLabel->hide();
-    buildEntitySummary();
-}
-
-void SelectedBuildingPanel::buildDebrisSingle()
-{
-    // "Debris" heading + a single "Scrap" stat row for the piece's remaining amount,
-    // mirroring the single-actor stats panels (REQ-UI-DEBRIS-PANEL).
-    m_entityTitleLabel->setText(tr("Debris"));
-    m_entityTitleLabel->show();
-    m_scrapLabel->setText(tr("Scrap: %1").arg(selectedDebrisScrapTotal()));
-    m_scrapLabel->show();
-}
-
-void SelectedBuildingPanel::buildEntitySummary()
-{
-    EntityAdmin& admin = m_sim->getAdmin();
-
-    // Group actors by faction + kind + ship schematic, preserving first-seen order
-    // (REQ-UI-FIELD-MULTI-SELECTION).
-    std::vector<QString> keys;
-    std::map<QString, int> counts;
-    std::map<QString, QString> labels;
-
-    for (entt::entity entity : m_selectedEntities)
-    {
-        if (!admin.isValid(entity)) { continue; }
-        const bool isEnemy = admin.hasAll<FactionComponent>(entity)
-            && admin.get<FactionComponent>(entity).isEnemy;
-
-        QString key;
-        QString label;
-        if (admin.hasAll<ShipIdentityComponent>(entity))
-        {
-            const std::string& id = admin.get<ShipIdentityComponent>(entity).schematicId;
-            const QString name = QString::fromStdString(toDisplayName(id));
-            key = (isEnemy ? QStringLiteral("ship:enemy:") : QStringLiteral("ship:player:"))
-                + QString::fromStdString(id);
-            label = isEnemy ? tr("Enemy %1").arg(name) : name;
-        }
-        else if (admin.hasAll<StationBodyComponent>(entity))
-        {
-            key = isEnemy ? QStringLiteral("station:enemy") : QStringLiteral("station:player");
-            label = isEnemy ? tr("Enemy Defence Station") : tr("Player Defence Station");
-        }
-        else
-        {
-            continue;
-        }
-
-        if (counts.find(key) == counts.end())
-        {
-            keys.push_back(key);
-            labels[key] = label;
-        }
-        counts[key] += 1;
-    }
-
-    // One "<type> x <count>" line per group (matching the recipe tooltip and the building
-    // multi-selection). No total-count header, consistent with the building panel. When
-    // debris is part of the selection, a "Debris x <count>" line followed by a
-    // "Scrap x <total>" line are appended into the same label so the line spacing is
-    // uniform (REQ-UI-FIELD-MULTI-SELECTION, REQ-UI-DEBRIS-PANEL).
-    QStringList lines;
-    for (const QString& key : keys)
-    {
-        lines << tr("%1 x %2").arg(labels[key]).arg(counts[key]);
-    }
-    if (!m_selectedDebris.empty())
-    {
-        lines << tr("Debris x %1").arg(static_cast<int>(m_selectedDebris.size()));
-        lines << scrapTotalText();
-    }
-    m_entitySummaryLabel->setText(lines.join('\n'));
-    m_entitySummaryLabel->show();
-}
-
-void SelectedBuildingPanel::buildEntityShip(entt::entity entity)
-{
-    EntityAdmin& admin = m_sim->getAdmin();
-    const ShipIdentityComponent& identity = admin.get<ShipIdentityComponent>(entity);
-    const HealthComponent& health = admin.get<HealthComponent>(entity);
-
-    m_entityTitleLabel->setText(tr("Ship: %1")
-        .arg(QString::fromStdString(identity.schematicId)));
-    m_entityTitleLabel->show();
-
-    const ShipStats stats = buildShipStatsFromEntity(admin, entity);
-    m_entityStatsPanel->refreshFromLive(stats, health.hp);
-    m_entityStatsPanel->setBehavior(
-        admin.get<SelectedBehaviorComponent>(entity).winner);
-    m_entityStatsPanel->setDebugDrawEnabled(m_debugDraw);
-
-    const ShipDef* schematicDef =
-        m_config->ships.findShipDef(identity.schematicId);
-    if (schematicDef)
-    {
-        const double threat = calculateShipThreatCost(
-            m_config->threatCosts, *m_config, schematicDef->id,
-            schematicDef->defaultModules);
-        m_entityStatsPanel->setThreatCost(threat);
-    }
-
-    m_entityStatsPanel->show();
-
-    m_stationStatsLabel->hide();
-}
-
-void SelectedBuildingPanel::buildEntityStation(entt::entity entity)
-{
-    EntityAdmin& admin = m_sim->getAdmin();
-    const HealthComponent& health = admin.get<HealthComponent>(entity);
-
-    const bool isEnemy = admin.hasAll<FactionComponent>(entity)
-        && admin.get<FactionComponent>(entity).isEnemy;
-    m_entityTitleLabel->setText(isEnemy
-        ? tr("Enemy Defence Station")
-        : tr("Player Defence Station"));
-    m_entityTitleLabel->show();
-
-    float totalDps = 0.0f;
-    float maxRange = 0.0f;
-    bool hasWeapons = false;
-
-    admin.forEach<ModuleOwnerComponent, WeaponComponent>(
-        [&](entt::entity /*child*/, const ModuleOwnerComponent& owner, const WeaponComponent& w)
-        {
-            if (owner.owner != entity) { return; }
-            hasWeapons = true;
-            totalDps += w.damage * w.fireRateHz;
-            if (w.range_tiles > maxRange) { maxRange = w.range_tiles; }
-        });
-
-    QString statsText = tr("HP: %1 / %2")
-        .arg(static_cast<int>(health.hp + 0.5f))
-        .arg(static_cast<int>(health.maxHp + 0.5f));
-
-    if (hasWeapons)
-    {
-        statsText += tr("\nDPS: %1").arg(QString::number(static_cast<double>(totalDps), 'f', 1));
-        statsText += tr("\nRange: %1 tiles").arg(QString::number(static_cast<double>(maxRange), 'f', 1));
-    }
-
-    m_stationStatsLabel->setText(statsText);
-    m_stationStatsLabel->show();
-
-    m_entityStatsPanel->hide();
-}
-
-void SelectedBuildingPanel::refreshEntityStats()
-{
-    // Only the single-actor stats panel needs a live refresh; the multi-actor summary is
-    // static counts, and GameWorldView prunes dead/despawned actors and re-emits the
-    // selection (REQ-UI-ENTITY-CLICK-SELECT), so the panel does not mutate it here.
-    if (m_selectedEntities.size() != 1) { return; }
-
-    EntityAdmin& admin = m_sim->getAdmin();
-    const entt::entity entity = m_selectedEntities.front();
-
-    if (!admin.isValid(entity) || !admin.hasAll<HealthComponent>(entity)) { return; }
-    const HealthComponent& health = admin.get<HealthComponent>(entity);
-    if (health.hp <= 0.0f) { return; }
-
-    if (admin.hasAll<ShipIdentityComponent>(entity))
-    {
-        const ShipStats stats = buildShipStatsFromEntity(admin, entity);
-        m_entityStatsPanel->refreshFromLive(stats, health.hp);
-        m_entityStatsPanel->setBehavior(
-            admin.get<SelectedBehaviorComponent>(entity).winner);
-    }
-    else if (admin.hasAll<StationBodyComponent>(entity))
-    {
-        buildEntityStation(entity);
-    }
-}
-
-void SelectedBuildingPanel::clearEntityDisplay()
-{
-    m_selectedEntities.clear();
-    m_entityTitleLabel->hide();
-    m_entityStatsPanel->hide();
-    m_stationStatsLabel->hide();
-    m_entitySummaryLabel->hide();
+    m_fieldSelectionPanel->setSelectedEntities(event->entities);
+    yieldToFieldSelection();
 }
 
 void SelectedBuildingPanel::handleEvent(std::shared_ptr<const SelectionChangedEvent> event)
@@ -1166,39 +863,9 @@ void SelectedBuildingPanel::handleEvent(std::shared_ptr<const SelectionChangedEv
 
 void SelectedBuildingPanel::handleEvent(
     std::shared_ptr<const DebrisSelectionChangedEvent> event)
-{
-    m_selectedDebris = event->debris;
-    if (!m_selectedDebris.empty())
 {
     // Debris is a field object: it supersedes any building selection but coexists
     // with actors (REQ-UI-SELECTION-CATEGORIES).
-        m_selectedBuildingIds.clear();
-    }
-    buildFieldSelection();
-}
-
-int SelectedBuildingPanel::selectedDebrisScrapTotal() const
-{
-    // Sum the remaining scrap across the still-living selected debris (REQ-UI-DEBRIS-PANEL).
-    int total = 0;
-    for (const DebrisInfo& info : m_sim->getDebrisSystem().getAllDebrisInfo())
-    {
-        if (std::find(m_selectedDebris.begin(), m_selectedDebris.end(), info.entity)
-            != m_selectedDebris.end())
-        {
-            total += info.amount;
-        }
-    }
-    return total;
-}
-
-QString SelectedBuildingPanel::scrapTotalText() const
-{
-    return tr("Scrap x %1").arg(selectedDebrisScrapTotal());
-}
-
-void SelectedBuildingPanel::handleEvent(std::shared_ptr<const DebugDrawToggledEvent> event)
-{
-    m_debugDraw = event->active;
-    m_entityStatsPanel->setDebugDrawEnabled(event->active);
+    m_fieldSelectionPanel->setSelectedDebris(event->debris);
+    yieldToFieldSelection();
 }

src/ui/SelectedBuildingPanel.h

@@ -7,12 +7,9 @@
 #include <QPoint>
 #include <QWidget>
 
-#include "entt/entity/entity.hpp"
-
 #include "BeltSystem.h"
 #include "Building.h"
 #include "BuildingId.h"
-#include "DebugDrawToggledEvent.h"
 #include "EntitySelectionChangedEvent.h"
 #include "EventHandler.h"
 #include "GameConfig.h"
@@ -26,20 +23,27 @@
 #include "TickAdvancedEvent.h"
 
 class Simulation;
+class FieldSelectionPanel;
 class ShipLayoutPreview;
-class ShipStatsPanel;
 class QLabel;
 class QListWidget;
 class QPushButton;
 class QVBoxLayout;
 
+// Shows the current selection. The building category (buildings and construction sites)
+// is rendered by this panel itself; the field category (ships, defence stations, debris)
+// is rendered by the embedded FieldSelectionPanel.
+//
+// The two categories are mutually exclusive (REQ-UI-SELECTION-CATEGORIES) and this panel
+// is the sole arbiter of which one owns the content: it listens to all three selection
+// events, forwards the field ones to the child panel, and drops the losing category's
+// content. Neither panel touches the other's widgets.
 class SelectedBuildingPanel : public QWidget,
                                public CombinedEventHandler<TickAdvancedEvent,
                                                            PlayerCommandsAppliedEvent,
                                                            EntitySelectionChangedEvent,
                                                            SelectionChangedEvent,
-                                                           DebrisSelectionChangedEvent,
-                                                           DebugDrawToggledEvent>
+                                                           DebrisSelectionChangedEvent>
 {
     Q_OBJECT
 
@@ -54,7 +58,6 @@ private:
     void handleEvent(std::shared_ptr<const EntitySelectionChangedEvent> event) override;
     void handleEvent(std::shared_ptr<const SelectionChangedEvent> event) override;
     void handleEvent(std::shared_ptr<const DebrisSelectionChangedEvent> event) override;
-    void handleEvent(std::shared_ptr<const DebugDrawToggledEvent> event) override;
 
 private slots:
     void onSelectRecipeClicked();
@@ -73,17 +76,14 @@ private:
     };
 
     void onSelectionChanged(const std::vector<BuildingId>& ids);
+    // Gives the panel to the field category once it has anything selected.
+    void yieldToFieldSelection();
     void refreshSelectionDisplay(RefreshReason reason);
     void rebuild();
     void hideAllWidgets();
-    void clearContent();
     void buildEmpty();
     void buildSingle(BuildingId id);
     void buildMulti(const std::vector<BuildingId>& ids);
-    // Summed remaining scrap across the selected debris (REQ-UI-DEBRIS-PANEL).
-    int selectedDebrisScrapTotal() const;
-    // "Scrap x N" line for the multi-object summary (REQ-UI-FIELD-MULTI-SELECTION).
-    QString scrapTotalText() const;
     void refreshBuffers(const Building* b);
     void refreshSiteProgress(const ConstructionSite* s);
     void updateShipyardLayoutWidgets(BuildingType type,
@@ -116,28 +116,7 @@ private:
     QPoint       m_splitterTile;
     std::string  m_currentRecipeId;
 
-    bool m_debugDraw = false;
-    // The selected ships/defence stations. Shares the "field" selection category with
-    // debris (m_selectedDebris): both can be non-empty at once (REQ-UI-SELECTION-CATEGORIES).
-    std::vector<entt::entity> m_selectedEntities;
-    ShipStatsPanel* m_entityStatsPanel;
-    QLabel*         m_entityTitleLabel;
-    QLabel*         m_stationStatsLabel;
-    QLabel*         m_entitySummaryLabel;
-
-    std::vector<entt::entity> m_selectedDebris;
-    // Shows the debris "Scrap" stat row (single selection) — the scrap total for the
-    // multi-object summary lives in m_entitySummaryLabel instead.
-    QLabel*                   m_scrapLabel;
-
-    // Renders the combined field selection (actors + debris): a single-object stats panel
-    // (ship, station, or debris) or a multi-object count summary that appends the debris
-    // count and scrap total when debris is also selected (REQ-UI-FIELD-MULTI-SELECTION).
-    void buildFieldSelection();
-    void buildEntityShip(entt::entity entity);
-    void buildEntityStation(entt::entity entity);
-    void buildEntitySummary();
-    void buildDebrisSingle();
-    void refreshEntityStats();
-    void clearEntityDisplay();
+    // Renders the field selection (actors + debris) below the building content
+    // (REQ-UI-FIELD-MULTI-SELECTION). Hides itself while nothing field-side is selected.
+    FieldSelectionPanel* m_fieldSelectionPanel;
 };