Unify dock leaves around single-tab stacks

new_editor/app/Shell/ProductShellAsset.cpp

@@ -40,7 +40,7 @@ UIEditorWorkspaceModel BuildWorkspace() {
             "workspace-top",
             UIEditorWorkspaceSplitAxis::Horizontal,
             0.15f,
-            BuildUIEditorWorkspacePanel(
+            BuildUIEditorWorkspaceSingleTabStack(
                 "hierarchy-panel",
                 "hierarchy",
                 "Hierarchy",
@@ -64,7 +64,7 @@ UIEditorWorkspaceModel BuildWorkspace() {
                             false)
                     },
                     0u),
-                BuildUIEditorWorkspacePanel(
+                BuildUIEditorWorkspaceSingleTabStack(
                     "inspector-panel",
                     "inspector",
                     "Inspector",

new_editor/include/XCEditor/Shell/UIEditorDockHost.h

@@ -96,15 +96,6 @@ struct UIEditorDockHostSplitterLayout {
     bool active = false;
 };
 
-struct UIEditorDockHostPanelLayout {
-    std::string nodeId = {};
-    std::string panelId = {};
-    std::string title = {};
-    bool active = false;
-    UIEditorPanelFrameState frameState = {};
-    UIEditorPanelFrameLayout frameLayout = {};
-};
-
 struct UIEditorDockHostTabStackLayout {
     std::string nodeId = {};
     std::string selectedPanelId = {};
@@ -119,7 +110,6 @@ struct UIEditorDockHostTabStackLayout {
 struct UIEditorDockHostLayout {
     ::XCEngine::UI::UIRect bounds = {};
     std::vector<UIEditorDockHostSplitterLayout> splitters = {};
-    std::vector<UIEditorDockHostPanelLayout> panels = {};
     std::vector<UIEditorDockHostTabStackLayout> tabStacks = {};
 };
 

new_editor/include/XCEditor/Shell/UIEditorWorkspaceModel.h

@@ -78,6 +78,12 @@ UIEditorWorkspaceNode BuildUIEditorWorkspacePanel(
     std::string title,
     bool placeholder = false);
 
+UIEditorWorkspaceNode BuildUIEditorWorkspaceSingleTabStack(
+    std::string nodeId,
+    std::string panelId,
+    std::string title,
+    bool placeholder = false);
+
 UIEditorWorkspaceNode BuildUIEditorWorkspaceTabStack(
     std::string nodeId,
     std::vector<UIEditorWorkspaceNode> panels,
@@ -93,6 +99,9 @@ UIEditorWorkspaceNode BuildUIEditorWorkspaceSplit(
 UIEditorWorkspaceValidationResult ValidateUIEditorWorkspace(
     const UIEditorWorkspaceModel& workspace);
 
+UIEditorWorkspaceModel CanonicalizeUIEditorWorkspaceModel(
+    UIEditorWorkspaceModel workspace);
+
 std::vector<UIEditorWorkspaceVisiblePanel> CollectUIEditorWorkspaceVisiblePanels(
     const UIEditorWorkspaceModel& workspace);
 

new_editor/src/Shell/UIEditorDockHost.cpp

@@ -23,9 +23,6 @@ using ::XCEngine::UI::Layout::UITabStripMeasureItem;
 using ::XCEngine::UI::Layout::UILayoutAxis;
 using ::XCEngine::UI::Widgets::ExpandUISplitterHandleHitRect;
 
-constexpr std::string_view kStandalonePanelActiveFooter = "";
-constexpr std::string_view kStandalonePanelInactiveFooter = "";
-
 struct DockMeasureResult {
     bool visible = false;
     UISize minimumSize = {};
@@ -102,17 +99,6 @@ UIEditorPanelFrameMetrics BuildTabContentFrameMetrics(
     return frameMetrics;
 }
 
-UISize MeasurePanelMinimumSize(
-    const UIEditorDockHostMetrics& metrics) {
-    const UIEditorPanelFrameMetrics& frameMetrics = metrics.panelFrameMetrics;
-    return UISize(
-        metrics.minimumStandalonePanelBodySize.width +
-            ClampNonNegative(frameMetrics.contentPadding) * 2.0f,
-        ClampNonNegative(frameMetrics.headerHeight) +
-            metrics.minimumStandalonePanelBodySize.height +
-            ClampNonNegative(frameMetrics.contentPadding) * 2.0f);
-}
-
 UISize MeasureTabContentMinimumSize(
     const UIEditorDockHostMetrics& metrics) {
     const UIEditorPanelFrameMetrics frameMetrics = BuildTabContentFrameMetrics(metrics);
@@ -205,14 +191,8 @@ DockMeasureResult MeasureNodeRecursive(
     const UIEditorWorkspaceSession& session,
     const UIEditorDockHostMetrics& metrics) {
     switch (node.kind) {
-    case UIEditorWorkspaceNodeKind::Panel: {
+    case UIEditorWorkspaceNodeKind::Panel:
-        DockMeasureResult result = {};
+        return {};
-        result.visible = IsPanelOpenAndVisible(session, node.panel.panelId);
-        if (result.visible) {
-            result.minimumSize = MeasurePanelMinimumSize(metrics);
-        }
-        return result;
-    }
     case UIEditorWorkspaceNodeKind::TabStack:
         return MeasureTabStackNode(node, panelRegistry, session, metrics);
     case UIEditorWorkspaceNodeKind::Split:
@@ -269,38 +249,6 @@ UIEditorTabStripState BuildTabStripState(
     return tabState;
 }
 
-UIEditorPanelFrameState BuildStandalonePanelFrameState(
-    const UIEditorDockHostState& state,
-    const UIEditorWorkspaceModel& workspace,
-    const UIEditorPanelDescriptor* descriptor,
-    const UIEditorWorkspaceNode& node) {
-    UIEditorPanelFrameState frameState = {};
-    frameState.active = workspace.activePanelId == node.panel.panelId;
-    frameState.focused = state.focused && frameState.active;
-    frameState.closable = descriptor != nullptr ? descriptor->canClose : true;
-    frameState.pinnable = false;
-    frameState.showFooter = true;
-
-    if (state.hoveredTarget.panelId != node.panel.panelId) {
-        return frameState;
-    }
-
-    switch (state.hoveredTarget.kind) {
-    case UIEditorDockHostHitTargetKind::PanelHeader:
-    case UIEditorDockHostHitTargetKind::PanelBody:
-    case UIEditorDockHostHitTargetKind::PanelFooter:
-    case UIEditorDockHostHitTargetKind::PanelCloseButton:
-        frameState.hovered = true;
-        break;
-    default:
-        break;
-    }
-
-    frameState.closeHovered =
-        state.hoveredTarget.kind == UIEditorDockHostHitTargetKind::PanelCloseButton;
-    return frameState;
-}
-
 UIEditorPanelFrameState BuildTabContentFrameState(
     const UIEditorDockHostState& state,
     const UIEditorWorkspaceModel& workspace,
@@ -338,29 +286,6 @@ void LayoutNodeRecursive(
     const UIEditorDockHostMetrics& metrics,
     UIEditorDockHostLayout& layout);
 
-void LayoutPanelNode(
-    const UIEditorWorkspaceNode& node,
-    const UIRect& bounds,
-    const UIEditorPanelRegistry& panelRegistry,
-    const UIEditorWorkspaceModel& workspace,
-    const UIEditorDockHostState& state,
-    const UIEditorDockHostMetrics& metrics,
-    UIEditorDockHostLayout& layout) {
-    const UIEditorPanelDescriptor* descriptor =
-        FindPanelDescriptor(panelRegistry, node.panel.panelId);
-
-    UIEditorDockHostPanelLayout panelLayout = {};
-    panelLayout.nodeId = node.nodeId;
-    panelLayout.panelId = node.panel.panelId;
-    panelLayout.title = node.panel.title;
-    panelLayout.active = workspace.activePanelId == node.panel.panelId;
-    panelLayout.frameState =
-        BuildStandalonePanelFrameState(state, workspace, descriptor, node);
-    panelLayout.frameLayout =
-        BuildUIEditorPanelFrameLayout(bounds, panelLayout.frameState, metrics.panelFrameMetrics);
-    layout.panels.push_back(std::move(panelLayout));
-}
-
 void LayoutTabStackNode(
     const UIEditorWorkspaceNode& node,
     const UIRect& bounds,
@@ -525,9 +450,6 @@ void LayoutNodeRecursive(
     UIEditorDockHostLayout& layout) {
     switch (node.kind) {
     case UIEditorWorkspaceNodeKind::Panel:
-        if (IsPanelOpenAndVisible(session, node.panel.panelId)) {
-            LayoutPanelNode(node, bounds, panelRegistry, workspace, state, metrics, layout);
-        }
         return;
     case UIEditorWorkspaceNodeKind::TabStack:
         LayoutTabStackNode(node, bounds, panelRegistry, workspace, session, state, metrics, layout);
@@ -606,11 +528,13 @@ UIEditorDockHostLayout BuildUIEditorDockHostLayout(
         bounds.y,
         ClampNonNegative(bounds.width),
         ClampNonNegative(bounds.height));
+    const UIEditorWorkspaceModel canonicalWorkspace =
+        CanonicalizeUIEditorWorkspaceModel(workspace);
     LayoutNodeRecursive(
-        workspace.root,
+        canonicalWorkspace.root,
         layout.bounds,
         panelRegistry,
-        workspace,
+        canonicalWorkspace,
         session,
         state,
         metrics,
@@ -675,17 +599,6 @@ UIEditorDockHostHitTarget HitTestUIEditorDockHost(
         }
     }
 
-    for (std::size_t index = layout.panels.size(); index > 0u; --index) {
-        const UIEditorDockHostPanelLayout& panel = layout.panels[index - 1u];
-        const UIEditorPanelFrameHitTarget hitTarget = HitTestUIEditorPanelFrame(
-            panel.frameLayout,
-            panel.frameState,
-            point);
-        if (hitTarget != UIEditorPanelFrameHitTarget::None) {
-            return MapPanelFrameHitTarget(hitTarget, panel.nodeId, panel.panelId);
-        }
-    }
-
     return {};
 }
 
@@ -694,15 +607,6 @@ void AppendUIEditorDockHostBackground(
     const UIEditorDockHostLayout& layout,
     const UIEditorDockHostPalette& palette,
     const UIEditorDockHostMetrics& metrics) {
-    for (const UIEditorDockHostPanelLayout& panel : layout.panels) {
-        AppendUIEditorPanelFrameBackground(
-            drawList,
-            panel.frameLayout,
-            panel.frameState,
-            palette.panelFramePalette,
-            metrics.panelFrameMetrics);
-    }
-
     const UIEditorPanelFrameMetrics tabContentFrameMetrics =
         BuildTabContentFrameMetrics(metrics);
     for (const UIEditorDockHostTabStackLayout& tabStack : layout.tabStacks) {
@@ -744,23 +648,6 @@ void AppendUIEditorDockHostForeground(
     const UIEditorDockHostForegroundOptions& options,
     const UIEditorDockHostPalette& palette,
     const UIEditorDockHostMetrics& metrics) {
-    for (const UIEditorDockHostPanelLayout& panel : layout.panels) {
-        AppendUIEditorPanelFrameForeground(
-            drawList,
-            panel.frameLayout,
-            panel.frameState,
-            UIEditorPanelFrameText{
-                panel.title,
-                {},
-                panel.active ? kStandalonePanelActiveFooter : kStandalonePanelInactiveFooter
-            },
-            palette.panelFramePalette,
-            metrics.panelFrameMetrics);
-        if (UsesExternalBodyPresentation(options, panel.panelId)) {
-            continue;
-        }
-    }
-
     const UIEditorPanelFrameMetrics tabContentFrameMetrics =
         BuildTabContentFrameMetrics(metrics);
     for (const UIEditorDockHostTabStackLayout& tabStack : layout.tabStacks) {

new_editor/src/Shell/UIEditorPanelContentHost.cpp

@@ -12,12 +12,6 @@ namespace {
 const ::XCEngine::UI::UIRect* FindVisiblePanelBodyRect(
     const Widgets::UIEditorDockHostLayout& layout,
     std::string_view panelId) {
-    for (const Widgets::UIEditorDockHostPanelLayout& panel : layout.panels) {
-        if (panel.panelId == panelId) {
-            return &panel.frameLayout.bodyRect;
-        }
-    }
-
     for (const Widgets::UIEditorDockHostTabStackLayout& tabStack : layout.tabStacks) {
         if (tabStack.selectedPanelId == panelId) {
             return &tabStack.contentFrameLayout.bodyRect;

new_editor/src/Shell/UIEditorWorkspaceController.cpp

@@ -75,9 +75,9 @@ UIEditorWorkspaceController::UIEditorWorkspaceController(
     UIEditorWorkspaceModel workspace,
     UIEditorWorkspaceSession session)
     : m_panelRegistry(std::move(panelRegistry))
-    , m_baselineWorkspace(workspace)
+    , m_baselineWorkspace(CanonicalizeUIEditorWorkspaceModel(workspace))
     , m_baselineSession(session)
-    , m_workspace(std::move(workspace))
+    , m_workspace(m_baselineWorkspace)
     , m_session(std::move(session)) {
 }
 
@@ -178,6 +178,10 @@ const UIEditorPanelDescriptor* UIEditorWorkspaceController::FindPanelDescriptor(
 
 UIEditorWorkspaceLayoutOperationResult UIEditorWorkspaceController::RestoreLayoutSnapshot(
     const UIEditorWorkspaceLayoutSnapshot& snapshot) {
+    UIEditorWorkspaceLayoutSnapshot canonicalSnapshot = snapshot;
+    canonicalSnapshot.workspace =
+        CanonicalizeUIEditorWorkspaceModel(std::move(canonicalSnapshot.workspace));
+
     const UIEditorPanelRegistryValidationResult registryValidation =
         ValidateUIEditorPanelRegistry(m_panelRegistry);
     if (!registryValidation.IsValid()) {
@@ -187,7 +191,7 @@ UIEditorWorkspaceLayoutOperationResult UIEditorWorkspaceController::RestoreLayou
     }
 
     const UIEditorWorkspaceValidationResult workspaceValidation =
-        ValidateUIEditorWorkspace(snapshot.workspace);
+        ValidateUIEditorWorkspace(canonicalSnapshot.workspace);
     if (!workspaceValidation.IsValid()) {
         return BuildLayoutOperationResult(
             UIEditorWorkspaceLayoutOperationStatus::Rejected,
@@ -195,15 +199,18 @@ UIEditorWorkspaceLayoutOperationResult UIEditorWorkspaceController::RestoreLayou
     }
 
     const UIEditorWorkspaceSessionValidationResult sessionValidation =
-        ValidateUIEditorWorkspaceSession(m_panelRegistry, snapshot.workspace, snapshot.session);
+        ValidateUIEditorWorkspaceSession(
+            m_panelRegistry,
+            canonicalSnapshot.workspace,
+            canonicalSnapshot.session);
     if (!sessionValidation.IsValid()) {
         return BuildLayoutOperationResult(
             UIEditorWorkspaceLayoutOperationStatus::Rejected,
             "Layout session invalid: " + sessionValidation.message);
     }
 
-    if (AreUIEditorWorkspaceModelsEquivalent(m_workspace, snapshot.workspace) &&
+    if (AreUIEditorWorkspaceModelsEquivalent(m_workspace, canonicalSnapshot.workspace) &&
-        AreUIEditorWorkspaceSessionsEquivalent(m_session, snapshot.session)) {
+        AreUIEditorWorkspaceSessionsEquivalent(m_session, canonicalSnapshot.session)) {
         return BuildLayoutOperationResult(
             UIEditorWorkspaceLayoutOperationStatus::NoOp,
             "Current state already matches the requested layout snapshot.");
@@ -211,8 +218,8 @@ UIEditorWorkspaceLayoutOperationResult UIEditorWorkspaceController::RestoreLayou
 
     const UIEditorWorkspaceModel previousWorkspace = m_workspace;
     const UIEditorWorkspaceSession previousSession = m_session;
-    m_workspace = snapshot.workspace;
+    m_workspace = canonicalSnapshot.workspace;
-    m_session = snapshot.session;
+    m_session = canonicalSnapshot.session;
 
     const UIEditorWorkspaceControllerValidationResult validation = ValidateState();
     if (!validation.IsValid()) {
@@ -441,10 +448,12 @@ UIEditorWorkspaceCommandResult UIEditorWorkspaceController::Dispatch(
 UIEditorWorkspaceController BuildDefaultUIEditorWorkspaceController(
     const UIEditorPanelRegistry& panelRegistry,
     const UIEditorWorkspaceModel& workspace) {
+    const UIEditorWorkspaceModel canonicalWorkspace =
+        CanonicalizeUIEditorWorkspaceModel(workspace);
     return UIEditorWorkspaceController(
         panelRegistry,
-        workspace,
+        canonicalWorkspace,
-        BuildDefaultUIEditorWorkspaceSession(panelRegistry, workspace));
+        BuildDefaultUIEditorWorkspaceSession(panelRegistry, canonicalWorkspace));
 }
 
 } // namespace XCEngine::UI::Editor

new_editor/src/Shell/UIEditorWorkspaceLayoutPersistence.cpp

@@ -391,7 +391,7 @@ UIEditorWorkspaceLayoutSnapshot BuildUIEditorWorkspaceLayoutSnapshot(
     const UIEditorWorkspaceModel& workspace,
     const UIEditorWorkspaceSession& session) {
     UIEditorWorkspaceLayoutSnapshot snapshot = {};
-    snapshot.workspace = workspace;
+    snapshot.workspace = CanonicalizeUIEditorWorkspaceModel(workspace);
     snapshot.session = session;
     return snapshot;
 }
@@ -461,6 +461,8 @@ UIEditorWorkspaceLayoutLoadResult DeserializeUIEditorWorkspaceLayoutSnapshot(
         return rootResult;
     }
 
+    snapshot.workspace = CanonicalizeUIEditorWorkspaceModel(std::move(snapshot.workspace));
+
     snapshot.session.panelStates.clear();
     while (index < lines.size()) {
         UIEditorPanelSessionState state = {};

new_editor/src/Shell/UIEditorWorkspaceModel.cpp

@@ -22,6 +22,55 @@ bool IsValidSplitRatio(float value) {
     return std::isfinite(value) && value > 0.0f && value < 1.0f;
 }
 
+std::string BuildSingleTabPanelNodeId(std::string_view stackNodeId) {
+    if (stackNodeId.empty()) {
+        return "single-tab-panel";
+    }
+
+    return std::string(stackNodeId) + "__panel";
+}
+
+UIEditorWorkspaceNode WrapStandalonePanelAsTabStack(UIEditorWorkspaceNode panelNode) {
+    UIEditorWorkspaceNode panelChild = std::move(panelNode);
+    const std::string stackNodeId = panelChild.nodeId;
+    panelChild.nodeId = BuildSingleTabPanelNodeId(stackNodeId);
+
+    UIEditorWorkspaceNode tabStack = {};
+    tabStack.kind = UIEditorWorkspaceNodeKind::TabStack;
+    tabStack.nodeId = stackNodeId;
+    tabStack.selectedTabIndex = 0u;
+    tabStack.children.push_back(std::move(panelChild));
+    return tabStack;
+}
+
+void CanonicalizeNodeRecursive(
+    UIEditorWorkspaceNode& node,
+    bool allowStandalonePanelLeaf) {
+    if (node.kind == UIEditorWorkspaceNodeKind::Panel) {
+        if (!allowStandalonePanelLeaf) {
+            node = WrapStandalonePanelAsTabStack(std::move(node));
+        }
+        return;
+    }
+
+    for (UIEditorWorkspaceNode& child : node.children) {
+        CanonicalizeNodeRecursive(
+            child,
+            node.kind == UIEditorWorkspaceNodeKind::TabStack);
+    }
+
+    if (node.kind == UIEditorWorkspaceNodeKind::TabStack &&
+        !node.children.empty() &&
+        node.selectedTabIndex >= node.children.size()) {
+        node.selectedTabIndex = node.children.size() - 1u;
+    }
+
+    if (node.kind == UIEditorWorkspaceNodeKind::Split &&
+        node.children.size() == 1u) {
+        node = std::move(node.children.front());
+    }
+}
+
 const UIEditorPanelDescriptor& RequirePanelDescriptor(
     const UIEditorPanelRegistry& registry,
     std::string_view panelId) {
@@ -274,7 +323,7 @@ UIEditorWorkspaceModel BuildDefaultEditorShellWorkspaceModel() {
         RequirePanelDescriptor(registry, "editor-foundation-root");
 
     UIEditorWorkspaceModel workspace = {};
-    workspace.root = BuildUIEditorWorkspacePanel(
+    workspace.root = BuildUIEditorWorkspaceSingleTabStack(
         "editor-foundation-root-node",
         rootPanel.panelId,
         rootPanel.defaultTitle,
@@ -297,6 +346,22 @@ UIEditorWorkspaceNode BuildUIEditorWorkspacePanel(
     return node;
 }
 
+UIEditorWorkspaceNode BuildUIEditorWorkspaceSingleTabStack(
+    std::string nodeId,
+    std::string panelId,
+    std::string title,
+    bool placeholder) {
+    UIEditorWorkspaceNode panel = BuildUIEditorWorkspacePanel(
+        BuildSingleTabPanelNodeId(nodeId),
+        std::move(panelId),
+        std::move(title),
+        placeholder);
+    return BuildUIEditorWorkspaceTabStack(
+        std::move(nodeId),
+        { std::move(panel) },
+        0u);
+}
+
 UIEditorWorkspaceNode BuildUIEditorWorkspaceTabStack(
     std::string nodeId,
     std::vector<UIEditorWorkspaceNode> panels,
@@ -345,6 +410,12 @@ UIEditorWorkspaceValidationResult ValidateUIEditorWorkspace(
     return {};
 }
 
+UIEditorWorkspaceModel CanonicalizeUIEditorWorkspaceModel(
+    UIEditorWorkspaceModel workspace) {
+    CanonicalizeNodeRecursive(workspace.root, false);
+    return workspace;
+}
+
 std::vector<UIEditorWorkspaceVisiblePanel> CollectUIEditorWorkspaceVisiblePanels(
     const UIEditorWorkspaceModel& workspace) {
     std::vector<UIEditorWorkspaceVisiblePanel> visiblePanels = {};

tests/UI/Editor/integration/shell/dock_host_basic/main.cpp

@@ -586,7 +586,7 @@ private:
 
         DrawCard(drawList, m_introRect, "这个测试验证什么功能？", "只验证 DockHost 基础交互 contract，不做 editor 业务面板。");
         drawList.AddText(UIPoint(m_introRect.x + 16.0f, m_introRect.y + 72.0f), "1. 验证 splitter drag 是否只通过 DockHostInteraction + WorkspaceController 完成。", kTextPrimary, 12.0f);
-        drawList.AddText(UIPoint(m_introRect.x + 16.0f, m_introRect.y + 94.0f), "2. 验证 tab activate / tab close / standalone panel activate / panel close。", kTextPrimary, 12.0f);
+        drawList.AddText(UIPoint(m_introRect.x + 16.0f, m_introRect.y + 94.0f), "2. 验证 unified dock：tab activate / tab close / single-tab body activate。", kTextPrimary, 12.0f);
         drawList.AddText(UIPoint(m_introRect.x + 16.0f, m_introRect.y + 116.0f), "3. 验证 active panel、visible panels、split ratio 是否统一收口到 controller。", kTextPrimary, 12.0f);
         drawList.AddText(UIPoint(m_introRect.x + 16.0f, m_introRect.y + 138.0f), "4. 验证 pointer capture / release 请求是否通过 contract 明确返回。", kTextPrimary, 12.0f);
         drawList.AddText(UIPoint(m_introRect.x + 16.0f, m_introRect.y + 162.0f), "建议操作：先拖中间 splitter，再点 Document A。", kTextWeak, 11.0f);

tests/UI/Editor/unit/test_ui_editor_dock_host.cpp

@@ -14,6 +14,7 @@ using XCEngine::UI::UIPoint;
 using XCEngine::UI::UIRect;
 using XCEngine::UI::Editor::BuildDefaultUIEditorWorkspaceSession;
 using XCEngine::UI::Editor::BuildUIEditorWorkspacePanel;
+using XCEngine::UI::Editor::BuildUIEditorWorkspaceSingleTabStack;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceSplit;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceTabStack;
 using XCEngine::UI::Editor::TryHideUIEditorWorkspacePanel;
@@ -62,8 +63,8 @@ UIEditorWorkspaceModel BuildWorkspace() {
             "right-split",
             UIEditorWorkspaceSplitAxis::Vertical,
             0.6f,
-            BuildUIEditorWorkspacePanel("details-node", "details", "Details", true),
+            BuildUIEditorWorkspaceSingleTabStack("details-node", "details", "Details", true),
-            BuildUIEditorWorkspacePanel("console-node", "console", "Console", true)));
+            BuildUIEditorWorkspaceSingleTabStack("console-node", "console", "Console", true)));
     workspace.activePanelId = "doc-b";
     return workspace;
 }
@@ -80,7 +81,7 @@ bool ContainsTextCommand(const UIDrawList& drawList, std::string_view text) {
 
 } // namespace
 
-TEST(UIEditorDockHostTest, LayoutComposesSplitTabStackAndStandalonePanelsFromWorkspaceTree) {
+TEST(UIEditorDockHostTest, LayoutComposesOnlyUnifiedTabStacksFromWorkspaceTree) {
     const UIEditorPanelRegistry registry = BuildPanelRegistry();
     const UIEditorWorkspaceModel workspace = BuildWorkspace();
     const UIEditorWorkspaceSession session =
@@ -93,8 +94,7 @@ TEST(UIEditorDockHostTest, LayoutComposesSplitTabStackAndStandalonePanelsFromWor
         session);
 
     ASSERT_EQ(layout.splitters.size(), 2u);
-    ASSERT_EQ(layout.tabStacks.size(), 1u);
+    ASSERT_EQ(layout.tabStacks.size(), 3u);
-    ASSERT_EQ(layout.panels.size(), 2u);
 
     const auto* rootSplitter = FindUIEditorDockHostSplitterLayout(layout, "root-split");
     ASSERT_NE(rootSplitter, nullptr);
@@ -109,8 +109,10 @@ TEST(UIEditorDockHostTest, LayoutComposesSplitTabStackAndStandalonePanelsFromWor
     EXPECT_EQ(tabStack.items[1].panelId, "doc-b");
     EXPECT_EQ(tabStack.tabStripState.selectedIndex, 1u);
 
-    EXPECT_EQ(layout.panels[0].panelId, "details");
+    EXPECT_EQ(layout.tabStacks[1].nodeId, "details-node");
-    EXPECT_EQ(layout.panels[1].panelId, "console");
+    EXPECT_EQ(layout.tabStacks[1].selectedPanelId, "details");
+    EXPECT_EQ(layout.tabStacks[2].nodeId, "console-node");
+    EXPECT_EQ(layout.tabStacks[2].selectedPanelId, "console");
 }
 
 TEST(UIEditorDockHostTest, HiddenBranchCollapsesAndVisibleBranchUsesFullBounds) {
@@ -130,7 +132,6 @@ TEST(UIEditorDockHostTest, HiddenBranchCollapsesAndVisibleBranchUsesFullBounds)
 
     EXPECT_TRUE(layout.splitters.empty());
     ASSERT_EQ(layout.tabStacks.size(), 1u);
-    EXPECT_TRUE(layout.panels.empty());
     EXPECT_FLOAT_EQ(layout.tabStacks.front().bounds.x, 10.0f);
     EXPECT_FLOAT_EQ(layout.tabStacks.front().bounds.y, 20.0f);
     EXPECT_FLOAT_EQ(layout.tabStacks.front().bounds.width, 640.0f);
@@ -158,7 +159,7 @@ TEST(UIEditorDockHostTest, HitTestPrioritizesSplitterThenTabCloseThenPanelBody)
     EXPECT_EQ(splitterHit.kind, UIEditorDockHostHitTargetKind::SplitterHandle);
     EXPECT_EQ(splitterHit.nodeId, "root-split");
 
-    ASSERT_EQ(layout.tabStacks.size(), 1u);
+    ASSERT_EQ(layout.tabStacks.size(), 3u);
     const auto& closeRect = layout.tabStacks.front().tabStripLayout.closeButtonRects[1];
     const auto tabCloseHit = HitTestUIEditorDockHost(
         layout,
@@ -209,7 +210,7 @@ TEST(UIEditorDockHostTest, BackgroundAndForegroundEmitStableCompositeCommands) {
     EXPECT_GT(foreground.GetCommandCount(), 10u);
 }
 
-TEST(UIEditorDockHostTest, ForegroundByDefaultStillDrawsPlaceholderText) {
+TEST(UIEditorDockHostTest, ForegroundDrawsUnifiedTabTitlesAcrossAllLeafStacks) {
     const UIEditorPanelRegistry registry = BuildPanelRegistry();
     const UIEditorWorkspaceModel workspace = BuildWorkspace();
     const UIEditorWorkspaceSession session =
@@ -224,11 +225,13 @@ TEST(UIEditorDockHostTest, ForegroundByDefaultStillDrawsPlaceholderText) {
     UIDrawList foreground("DockHostForegroundDefault");
     AppendUIEditorDockHostForeground(foreground, layout);
 
-    EXPECT_TRUE(ContainsTextCommand(foreground, "DockHost tab content placeholder"));
+    EXPECT_TRUE(ContainsTextCommand(foreground, "Document A"));
-    EXPECT_TRUE(ContainsTextCommand(foreground, "DockHost standalone panel"));
+    EXPECT_TRUE(ContainsTextCommand(foreground, "Document B"));
+    EXPECT_TRUE(ContainsTextCommand(foreground, "Details"));
+    EXPECT_TRUE(ContainsTextCommand(foreground, "Console"));
 }
 
-TEST(UIEditorDockHostTest, ForegroundSkipsPlaceholderForExternalBodyPanelId) {
+TEST(UIEditorDockHostTest, ForegroundWithExternalBodyStillDrawsUnifiedTabTitles) {
     const UIEditorPanelRegistry registry = BuildPanelRegistry();
     const UIEditorWorkspaceModel workspace = BuildWorkspace();
     const UIEditorWorkspaceSession session =
@@ -245,6 +248,8 @@ TEST(UIEditorDockHostTest, ForegroundSkipsPlaceholderForExternalBodyPanelId) {
     options.externalBodyPanelIds = { "doc-b" };
     AppendUIEditorDockHostForeground(foreground, layout, options);
 
-    EXPECT_FALSE(ContainsTextCommand(foreground, "DockHost tab content placeholder"));
+    EXPECT_TRUE(ContainsTextCommand(foreground, "Document A"));
-    EXPECT_TRUE(ContainsTextCommand(foreground, "DockHost standalone panel"));
+    EXPECT_TRUE(ContainsTextCommand(foreground, "Document B"));
+    EXPECT_TRUE(ContainsTextCommand(foreground, "Details"));
+    EXPECT_TRUE(ContainsTextCommand(foreground, "Console"));
 }

tests/UI/Editor/unit/test_ui_editor_dock_host_interaction.cpp

@@ -15,6 +15,7 @@ using XCEngine::UI::UIPointerButton;
 using XCEngine::Input::KeyCode;
 using XCEngine::UI::Editor::BuildDefaultUIEditorWorkspaceController;
 using XCEngine::UI::Editor::BuildUIEditorWorkspacePanel;
+using XCEngine::UI::Editor::BuildUIEditorWorkspaceSingleTabStack;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceSplit;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceTabStack;
 using XCEngine::UI::Editor::FindUIEditorPanelSessionState;
@@ -24,6 +25,8 @@ using XCEngine::UI::Editor::UIEditorWorkspaceModel;
 using XCEngine::UI::Editor::UIEditorWorkspaceSplitAxis;
 using XCEngine::UI::Editor::UpdateUIEditorDockHostInteraction;
 using XCEngine::UI::Editor::Widgets::UIEditorDockHostHitTargetKind;
+using XCEngine::UI::Editor::Widgets::UIEditorDockHostLayout;
+using XCEngine::UI::Editor::Widgets::UIEditorDockHostTabStackLayout;
 
 UIEditorPanelRegistry BuildPanelRegistry() {
     UIEditorPanelRegistry registry = {};
@@ -53,8 +56,8 @@ UIEditorWorkspaceModel BuildWorkspace() {
             "right-split",
             UIEditorWorkspaceSplitAxis::Vertical,
             0.6f,
-            BuildUIEditorWorkspacePanel("details-node", "details", "Details", true),
+            BuildUIEditorWorkspaceSingleTabStack("details-node", "details", "Details", true),
-            BuildUIEditorWorkspacePanel("console-node", "console", "Console", true)));
+            BuildUIEditorWorkspaceSingleTabStack("console-node", "console", "Console", true)));
     workspace.activePanelId = "doc-b";
     return workspace;
 }
@@ -99,6 +102,18 @@ UIPoint RectCenter(const UIRect& rect) {
     return UIPoint(rect.x + rect.width * 0.5f, rect.y + rect.height * 0.5f);
 }
 
+const UIEditorDockHostTabStackLayout* FindTabStackByNodeId(
+    const UIEditorDockHostLayout& layout,
+    std::string_view nodeId) {
+    for (const UIEditorDockHostTabStackLayout& tabStack : layout.tabStacks) {
+        if (tabStack.nodeId == nodeId) {
+            return &tabStack;
+        }
+    }
+
+    return nullptr;
+}
+
 } // namespace
 
 TEST(UIEditorDockHostInteractionTest, SplitterDragUpdatesWorkspaceSplitRatio) {
@@ -182,8 +197,9 @@ TEST(UIEditorDockHostInteractionTest, ClickingTabActivatesTargetPanel) {
         controller,
         UIRect(0.0f, 0.0f, 800.0f, 600.0f),
         {});
-    ASSERT_EQ(frame.layout.tabStacks.size(), 1u);
+    const auto* documentStack = FindTabStackByNodeId(frame.layout, "document-tabs");
-    const UIRect docARect = frame.layout.tabStacks.front().tabStripLayout.tabHeaderRects[0];
+    ASSERT_NE(documentStack, nullptr);
+    const UIRect docARect = documentStack->tabStripLayout.tabHeaderRects[0];
 
     frame = UpdateUIEditorDockHostInteraction(
         state,
@@ -209,8 +225,9 @@ TEST(UIEditorDockHostInteractionTest, ClickingTabActivatesTargetPanel) {
     EXPECT_TRUE(frame.result.consumed);
     EXPECT_TRUE(frame.result.commandExecuted);
     EXPECT_EQ(controller.GetWorkspace().activePanelId, "doc-a");
-    ASSERT_EQ(frame.layout.tabStacks.size(), 1u);
+    documentStack = FindTabStackByNodeId(frame.layout, "document-tabs");
-    EXPECT_EQ(frame.layout.tabStacks.front().selectedPanelId, "doc-a");
+    ASSERT_NE(documentStack, nullptr);
+    EXPECT_EQ(documentStack->selectedPanelId, "doc-a");
 }
 
 TEST(UIEditorDockHostInteractionTest, ClickingTabCloseClosesPanelThroughController) {
@@ -223,8 +240,9 @@ TEST(UIEditorDockHostInteractionTest, ClickingTabCloseClosesPanelThroughControll
         controller,
         UIRect(0.0f, 0.0f, 800.0f, 600.0f),
         {});
-    ASSERT_EQ(frame.layout.tabStacks.size(), 1u);
+    const auto* documentStack = FindTabStackByNodeId(frame.layout, "document-tabs");
-    const UIRect closeRect = frame.layout.tabStacks.front().tabStripLayout.closeButtonRects[1];
+    ASSERT_NE(documentStack, nullptr);
+    const UIRect closeRect = documentStack->tabStripLayout.closeButtonRects[1];
     const UIPoint closeCenter = RectCenter(closeRect);
 
     frame = UpdateUIEditorDockHostInteraction(
@@ -243,6 +261,14 @@ TEST(UIEditorDockHostInteractionTest, ClickingTabCloseClosesPanelThroughControll
     EXPECT_TRUE(frame.result.consumed);
     EXPECT_FALSE(frame.result.commandExecuted);
 
+    frame = UpdateUIEditorDockHostInteraction(
+        state,
+        controller,
+        UIRect(0.0f, 0.0f, 800.0f, 600.0f),
+        { MakePointerDown(closeCenter.x, closeCenter.y) });
+    EXPECT_TRUE(frame.result.consumed);
+    EXPECT_FALSE(frame.result.commandExecuted);
+
     frame = UpdateUIEditorDockHostInteraction(
         state,
         controller,
@@ -268,8 +294,9 @@ TEST(UIEditorDockHostInteractionTest, FocusedTabStripHandlesKeyboardNavigationTh
         controller,
         UIRect(0.0f, 0.0f, 800.0f, 600.0f),
         {});
-    ASSERT_EQ(frame.layout.tabStacks.size(), 1u);
+    const auto* documentStack = FindTabStackByNodeId(frame.layout, "document-tabs");
-    const UIRect docARect = frame.layout.tabStacks.front().tabStripLayout.tabHeaderRects[0];
+    ASSERT_NE(documentStack, nullptr);
+    const UIRect docARect = documentStack->tabStripLayout.tabHeaderRects[0];
     const UIPoint docACenter = RectCenter(docARect);
 
     frame = UpdateUIEditorDockHostInteraction(
@@ -302,8 +329,9 @@ TEST(UIEditorDockHostInteractionTest, FocusedTabStripHandlesKeyboardNavigationTh
     EXPECT_TRUE(frame.result.consumed);
     EXPECT_TRUE(frame.result.commandExecuted);
     EXPECT_EQ(controller.GetWorkspace().activePanelId, "doc-b");
-    ASSERT_EQ(frame.layout.tabStacks.size(), 1u);
+    documentStack = FindTabStackByNodeId(frame.layout, "document-tabs");
-    EXPECT_EQ(frame.layout.tabStacks.front().selectedPanelId, "doc-b");
+    ASSERT_NE(documentStack, nullptr);
+    EXPECT_EQ(documentStack->selectedPanelId, "doc-b");
 }
 
 TEST(UIEditorDockHostInteractionTest, BatchedPointerMoveDownUpActivatesTabInSameUpdateCall) {
@@ -316,9 +344,10 @@ TEST(UIEditorDockHostInteractionTest, BatchedPointerMoveDownUpActivatesTabInSame
         controller,
         UIRect(0.0f, 0.0f, 800.0f, 600.0f),
         {});
-    ASSERT_EQ(frame.layout.tabStacks.size(), 1u);
+    const auto* documentStack = FindTabStackByNodeId(frame.layout, "document-tabs");
+    ASSERT_NE(documentStack, nullptr);
     const UIPoint docACenter =
-        RectCenter(frame.layout.tabStacks.front().tabStripLayout.tabHeaderRects[0]);
+        RectCenter(documentStack->tabStripLayout.tabHeaderRects[0]);
 
     frame = UpdateUIEditorDockHostInteraction(
         state,
@@ -332,11 +361,12 @@ TEST(UIEditorDockHostInteractionTest, BatchedPointerMoveDownUpActivatesTabInSame
     EXPECT_TRUE(frame.result.consumed);
     EXPECT_TRUE(frame.result.commandExecuted);
     EXPECT_EQ(controller.GetWorkspace().activePanelId, "doc-a");
-    ASSERT_EQ(frame.layout.tabStacks.size(), 1u);
+    documentStack = FindTabStackByNodeId(frame.layout, "document-tabs");
-    EXPECT_EQ(frame.layout.tabStacks.front().selectedPanelId, "doc-a");
+    ASSERT_NE(documentStack, nullptr);
+    EXPECT_EQ(documentStack->selectedPanelId, "doc-a");
 }
 
-TEST(UIEditorDockHostInteractionTest, ClickingStandalonePanelBodyActivatesTargetPanel) {
+TEST(UIEditorDockHostInteractionTest, ClickingSingleTabStackBodyActivatesTargetPanel) {
     auto controller =
         BuildDefaultUIEditorWorkspaceController(BuildPanelRegistry(), BuildWorkspace());
     UIEditorDockHostInteractionState state = {};
@@ -346,8 +376,9 @@ TEST(UIEditorDockHostInteractionTest, ClickingStandalonePanelBodyActivatesTarget
         controller,
         UIRect(0.0f, 0.0f, 800.0f, 600.0f),
         {});
-    ASSERT_EQ(frame.layout.panels.size(), 2u);
+    const auto* detailsStack = FindTabStackByNodeId(frame.layout, "details-node");
-    const UIRect detailsBodyRect = frame.layout.panels[0].frameLayout.bodyRect;
+    ASSERT_NE(detailsStack, nullptr);
+    const UIRect detailsBodyRect = detailsStack->contentFrameLayout.bodyRect;
     const UIPoint detailsBodyCenter = RectCenter(detailsBodyRect);
 
     frame = UpdateUIEditorDockHostInteraction(
@@ -368,7 +399,7 @@ TEST(UIEditorDockHostInteractionTest, ClickingStandalonePanelBodyActivatesTarget
     EXPECT_EQ(controller.GetWorkspace().activePanelId, "details");
 }
 
-TEST(UIEditorDockHostInteractionTest, ClickingStandalonePanelCloseClosesPanelThroughController) {
+TEST(UIEditorDockHostInteractionTest, ClickingSingleTabStackTabCloseClosesPanelThroughController) {
     auto controller =
         BuildDefaultUIEditorWorkspaceController(BuildPanelRegistry(), BuildWorkspace());
     UIEditorDockHostInteractionState state = {};
@@ -378,8 +409,9 @@ TEST(UIEditorDockHostInteractionTest, ClickingStandalonePanelCloseClosesPanelThr
         controller,
         UIRect(0.0f, 0.0f, 800.0f, 600.0f),
         {});
-    ASSERT_EQ(frame.layout.panels.size(), 2u);
+    const auto* consoleStack = FindTabStackByNodeId(frame.layout, "console-node");
-    const UIRect closeRect = frame.layout.panels[1].frameLayout.closeButtonRect;
+    ASSERT_NE(consoleStack, nullptr);
+    const UIRect closeRect = consoleStack->tabStripLayout.closeButtonRects[0];
     const UIPoint closeCenter = RectCenter(closeRect);
 
     frame = UpdateUIEditorDockHostInteraction(
@@ -387,7 +419,7 @@ TEST(UIEditorDockHostInteractionTest, ClickingStandalonePanelCloseClosesPanelThr
         controller,
         UIRect(0.0f, 0.0f, 800.0f, 600.0f),
         { MakePointerMove(closeCenter.x, closeCenter.y) });
-    EXPECT_EQ(frame.result.hitTarget.kind, UIEditorDockHostHitTargetKind::PanelCloseButton);
+    EXPECT_EQ(frame.result.hitTarget.kind, UIEditorDockHostHitTargetKind::TabCloseButton);
     EXPECT_EQ(frame.result.hitTarget.panelId, "console");
 
     frame = UpdateUIEditorDockHostInteraction(

tests/UI/Editor/unit/test_ui_editor_panel_content_host.cpp

@@ -9,6 +9,7 @@ namespace {
 using XCEngine::UI::UIRect;
 using XCEngine::UI::Editor::BuildDefaultUIEditorWorkspaceSession;
 using XCEngine::UI::Editor::BuildUIEditorWorkspacePanel;
+using XCEngine::UI::Editor::BuildUIEditorWorkspaceSingleTabStack;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceSplit;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceTabStack;
 using XCEngine::UI::Editor::CollectMountedUIEditorPanelContentHostPanelIds;
@@ -50,7 +51,7 @@ UIEditorWorkspaceModel BuildWorkspace() {
                 BuildUIEditorWorkspacePanel("console-node", "console", "Console", true)
             },
             1u),
-        BuildUIEditorWorkspacePanel("inspector-node", "inspector", "Inspector"));
+        BuildUIEditorWorkspaceSingleTabStack("inspector-node", "inspector", "Inspector"));
     workspace.activePanelId = "doc-b";
     return workspace;
 }

tests/UI/Editor/unit/test_ui_editor_workspace_layout_persistence.cpp

@@ -14,6 +14,7 @@ using XCEngine::UI::Editor::BuildDefaultUIEditorWorkspaceController;
 using XCEngine::UI::Editor::BuildDefaultUIEditorWorkspaceSession;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceLayoutSnapshot;
 using XCEngine::UI::Editor::BuildUIEditorWorkspacePanel;
+using XCEngine::UI::Editor::BuildUIEditorWorkspaceSingleTabStack;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceSplit;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceTabStack;
 using XCEngine::UI::Editor::DeserializeUIEditorWorkspaceLayoutSnapshot;
@@ -28,6 +29,7 @@ using XCEngine::UI::Editor::UIEditorWorkspaceCommandKind;
 using XCEngine::UI::Editor::UIEditorWorkspaceCommandStatus;
 using XCEngine::UI::Editor::UIEditorWorkspaceModel;
 using XCEngine::UI::Editor::UIEditorWorkspaceSession;
+using XCEngine::UI::Editor::UIEditorWorkspaceNodeKind;
 using XCEngine::UI::Editor::UIEditorWorkspaceSplitAxis;
 
 UIEditorPanelRegistry BuildPanelRegistry() {
@@ -53,7 +55,7 @@ UIEditorWorkspaceModel BuildWorkspace() {
                 BuildUIEditorWorkspacePanel("doc-b-node", "doc-b", "Document B", true)
             },
             0u),
-        BuildUIEditorWorkspacePanel("details-node", "details", "Details", true));
+        BuildUIEditorWorkspaceSingleTabStack("details-node", "details", "Details", true));
     workspace.activePanelId = "doc-a";
     return workspace;
 }
@@ -128,6 +130,35 @@ TEST(UIEditorWorkspaceLayoutPersistenceTest, DeserializeRejectsMissingSessionRec
     EXPECT_EQ(loadResult.code, UIEditorWorkspaceLayoutLoadCode::InvalidWorkspaceSession);
 }
 
+TEST(UIEditorWorkspaceLayoutPersistenceTest, DeserializeUpgradesLegacyStandalonePanelLeaves) {
+    const UIEditorPanelRegistry registry = BuildPanelRegistry();
+    const std::string legacySerialized =
+        "XCUI_EDITOR_WORKSPACE_LAYOUT 1\n"
+        "active \"doc-a\"\n"
+        "node_split \"root-split\" \"horizontal\" 0.66\n"
+        "node_tabstack \"document-tabs\" 0 2\n"
+        "node_panel \"doc-a-node\" \"doc-a\" \"Document A\" 1\n"
+        "node_panel \"doc-b-node\" \"doc-b\" \"Document B\" 1\n"
+        "node_panel \"details-node\" \"details\" \"Details\" 1\n"
+        "session \"doc-a\" 1 1\n"
+        "session \"doc-b\" 1 1\n"
+        "session \"details\" 1 1\n";
+
+    const auto loadResult =
+        DeserializeUIEditorWorkspaceLayoutSnapshot(registry, legacySerialized);
+
+    ASSERT_TRUE(loadResult.IsValid()) << loadResult.message;
+    ASSERT_EQ(loadResult.snapshot.workspace.root.kind, UIEditorWorkspaceNodeKind::Split);
+    ASSERT_EQ(loadResult.snapshot.workspace.root.children.size(), 2u);
+    EXPECT_EQ(
+        loadResult.snapshot.workspace.root.children[1].kind,
+        UIEditorWorkspaceNodeKind::TabStack);
+    ASSERT_EQ(loadResult.snapshot.workspace.root.children[1].children.size(), 1u);
+    EXPECT_EQ(
+        loadResult.snapshot.workspace.root.children[1].children[0].panel.panelId,
+        "details");
+}
+
 TEST(UIEditorWorkspaceLayoutPersistenceTest, RestoreSerializedLayoutRestoresSavedStateAfterFurtherMutations) {
     const UIEditorPanelRegistry registry = BuildPanelRegistry();
     UIEditorWorkspaceController controller =

tests/UI/Editor/unit/test_ui_editor_workspace_model.cpp

@@ -9,8 +9,10 @@
 namespace {
 
 using XCEngine::UI::Editor::BuildUIEditorWorkspacePanel;
+using XCEngine::UI::Editor::BuildUIEditorWorkspaceSingleTabStack;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceSplit;
 using XCEngine::UI::Editor::BuildUIEditorWorkspaceTabStack;
+using XCEngine::UI::Editor::CanonicalizeUIEditorWorkspaceModel;
 using XCEngine::UI::Editor::CollectUIEditorWorkspaceVisiblePanels;
 using XCEngine::UI::Editor::ContainsUIEditorWorkspacePanel;
 using XCEngine::UI::Editor::FindUIEditorWorkspaceActivePanel;
@@ -73,7 +75,7 @@ TEST(UIEditorWorkspaceModelTest, VisiblePanelsOnlyIncludeSelectedTabsAcrossSplit
         "root-split",
         UIEditorWorkspaceSplitAxis::Horizontal,
         0.68f,
-        BuildUIEditorWorkspacePanel("left-panel-node", "left-panel", "Left Panel", true),
+        BuildUIEditorWorkspaceSingleTabStack("left-panel-node", "left-panel", "Left Panel", true),
         BuildUIEditorWorkspaceSplit(
             "right-split",
             UIEditorWorkspaceSplitAxis::Vertical,
@@ -85,7 +87,11 @@ TEST(UIEditorWorkspaceModelTest, VisiblePanelsOnlyIncludeSelectedTabsAcrossSplit
                     BuildUIEditorWorkspacePanel("doc-b-node", "doc-b", "Document B", true)
                 },
                 1u),
-            BuildUIEditorWorkspacePanel("bottom-panel-node", "bottom-panel", "Bottom Panel", true)));
+            BuildUIEditorWorkspaceSingleTabStack(
+                "bottom-panel-node",
+                "bottom-panel",
+                "Bottom Panel",
+                true)));
     workspace.activePanelId = "doc-b";
 
     const auto validation = ValidateUIEditorWorkspace(workspace);
@@ -116,7 +122,7 @@ TEST(UIEditorWorkspaceModelTest, ActivatingHiddenPanelSelectsContainingTabAndUpd
                 BuildUIEditorWorkspacePanel("doc-c-node", "doc-c", "Document C", true)
             },
             0u),
-        BuildUIEditorWorkspacePanel("details-node", "details", "Details", true));
+        BuildUIEditorWorkspaceSingleTabStack("details-node", "details", "Details", true));
 
     ASSERT_TRUE(ContainsUIEditorWorkspacePanel(workspace, "doc-b"));
     ASSERT_TRUE(TryActivateUIEditorWorkspacePanel(workspace, "doc-b"));
@@ -150,8 +156,8 @@ TEST(UIEditorWorkspaceModelTest, SplitRatioMutationTargetsSplitNodeAndRejectsInv
         "root-split",
         UIEditorWorkspaceSplitAxis::Horizontal,
         0.62f,
-        BuildUIEditorWorkspacePanel("left-node", "left", "Left", true),
+        BuildUIEditorWorkspaceSingleTabStack("left-node", "left", "Left", true),
-        BuildUIEditorWorkspacePanel("right-node", "right", "Right", true));
+        BuildUIEditorWorkspaceSingleTabStack("right-node", "right", "Right", true));
 
     ASSERT_TRUE(TrySetUIEditorWorkspaceSplitRatio(workspace, "root-split", 0.35f));
     const auto* splitNode = FindUIEditorWorkspaceNode(workspace, "root-split");
@@ -163,3 +169,29 @@ TEST(UIEditorWorkspaceModelTest, SplitRatioMutationTargetsSplitNodeAndRejectsInv
     EXPECT_FALSE(TrySetUIEditorWorkspaceSplitRatio(workspace, "missing", 0.5f));
     EXPECT_FALSE(TrySetUIEditorWorkspaceSplitRatio(workspace, "root-split", 1.0f));
 }
+
+TEST(UIEditorWorkspaceModelTest, CanonicalizeWrapsStandalonePanelsIntoSingleTabStacks) {
+    UIEditorWorkspaceModel workspace = {};
+    workspace.root = BuildUIEditorWorkspaceSplit(
+        "root-split",
+        UIEditorWorkspaceSplitAxis::Horizontal,
+        0.5f,
+        BuildUIEditorWorkspacePanel("left-node", "left", "Left", true),
+        BuildUIEditorWorkspacePanel("right-node", "right", "Right", true));
+
+    const UIEditorWorkspaceModel canonicalWorkspace =
+        CanonicalizeUIEditorWorkspaceModel(workspace);
+
+    ASSERT_EQ(canonicalWorkspace.root.kind, UIEditorWorkspaceNodeKind::Split);
+    ASSERT_EQ(canonicalWorkspace.root.children.size(), 2u);
+    EXPECT_EQ(canonicalWorkspace.root.children[0].kind, UIEditorWorkspaceNodeKind::TabStack);
+    EXPECT_EQ(canonicalWorkspace.root.children[0].nodeId, "left-node");
+    ASSERT_EQ(canonicalWorkspace.root.children[0].children.size(), 1u);
+    EXPECT_EQ(
+        canonicalWorkspace.root.children[0].children[0].kind,
+        UIEditorWorkspaceNodeKind::Panel);
+    EXPECT_EQ(
+        canonicalWorkspace.root.children[0].children[0].panel.panelId,
+        "left");
+    EXPECT_EQ(canonicalWorkspace.root.children[1].kind, UIEditorWorkspaceNodeKind::TabStack);
+}