feat(docking): DockBuilder surface + real split-layout demo

The previous demo pre-docked both panels into one tab group — visually just a
tab bar, no visible docking. Export the minimal DockBuilder surface
(DockBuilderSplitNode/DockWindow/Finish + ImGuiDir-typed Dir_* constants,
gated by the docking feature; imgui_internal.h enters the GMF only when the
feature is on) and rebuild the example as an IDE-style first-frame layout:
Scene | Viewport | Inspector with Console split below — four real dock nodes,
drag any tab to re-split/stack/float. Default (feature-off) build/tests
unchanged.

Author: d2learn-org

examples/docking/src/main.cpp

@@ -1,26 +1,49 @@
 // Docking demo — `imgui = { ..., features = ["docking"] }`.
-// The imgui.app facade auto-enables ImGuiConfigFlags_DockingEnable when the
-// feature is active; DockSpaceOverViewport turns the whole window into a
-// dock host, so the two panels below can be dragged into / split around it.
+//
+// On the first frame, DockBuilder splits the dockspace into an IDE-style
+// layout (Scene | Viewport | Inspector, Console at the bottom). Every pane is
+// a real dock node: drag any tab to re-split, stack, or float it — the
+// docking previews/overlays appear while dragging.
 import imgui.core;
 import imgui.app;
 
 int main() {
     return ImGui::App::run({.title = "mcpp imgui docking demo"}, [] {
-        const auto dockspace = ImGui::DockSpaceOverViewport();
+        auto dockspace = ImGui::DockSpaceOverViewport();
 
-        // Pre-dock both panels into the dockspace on first run; they remain
-        // freely draggable/splittable afterwards.
-        ImGui::SetNextWindowDockID(dockspace, ImGui::Cond_FirstUseEver);
-        ImGui::Begin("Toolbox");
-        ImGui::TextUnformatted("Drag this panel onto the dockspace.");
-        ImGui::TextUnformatted("Docked: ");
-        ImGui::TextUnformatted(ImGui::IsWindowDocked() ? "yes" : "no");
+        static bool layout_built = false;
+        if (!layout_built) {
+            layout_built = true;
+            const auto root = dockspace;
+            auto left  = ImGui::DockBuilderSplitNode(
+                dockspace, ImGui::Dir_Left, 0.22f, nullptr, &dockspace);
+            auto down  = ImGui::DockBuilderSplitNode(
+                dockspace, ImGui::Dir_Down, 0.28f, nullptr, &dockspace);
+            auto right = ImGui::DockBuilderSplitNode(
+                dockspace, ImGui::Dir_Right, 0.30f, nullptr, &dockspace);
+            ImGui::DockBuilderDockWindow("Scene",     left);
+            ImGui::DockBuilderDockWindow("Console",   down);
+            ImGui::DockBuilderDockWindow("Inspector", right);
+            ImGui::DockBuilderDockWindow("Viewport",  dockspace);
+            ImGui::DockBuilderFinish(root);
+        }
+
+        ImGui::Begin("Scene");
+        ImGui::TextUnformatted("Scene tree pane (left split).");
+        ImGui::TextUnformatted(ImGui::IsWindowDocked() ? "docked: yes" : "docked: no");
+        ImGui::End();
+
+        ImGui::Begin("Viewport");
+        ImGui::TextUnformatted("Central pane. Drag any tab to");
+        ImGui::TextUnformatted("re-split / stack / float it.");
         ImGui::End();
 
-        ImGui::SetNextWindowDockID(dockspace, ImGui::Cond_FirstUseEver);
         ImGui::Begin("Inspector");
-        ImGui::TextUnformatted("Split me against the Toolbox.");
+        ImGui::TextUnformatted("Properties pane (right split).");
+        ImGui::End();
+
+        ImGui::Begin("Console");
+        ImGui::TextUnformatted("Log pane (bottom split).");
         ImGui::End();
     });
 }

src/core.cppm

@@ -1,6 +1,10 @@
 module;
 
 #include <imgui.h>
+#ifdef MCPP_FEATURE_DOCKING
+// DockBuilder (programmatic dock layouts) lives in the internal header.
+#include <imgui_internal.h>
+#endif
 
 export module imgui.core;
 
@@ -42,9 +46,22 @@ export namespace ImGui {
     using ::ImGui::GetWindowDockID;
     using ::ImGui::IsWindowDocked;
 
+    // DockBuilder — programmatic dock layouts (e.g. an IDE-style default
+    // split). Upstream ships it in imgui_internal.h; the docking feature
+    // exports the minimal stable surface needed to build layouts.
+    using ::ImGui::DockBuilderSplitNode;
+    using ::ImGui::DockBuilderDockWindow;
+    using ::ImGui::DockBuilderFinish;
+
     // Unscoped global enumerators, re-exported as namespaced constants for
     // module consumers.
     inline constexpr int ConfigFlags_DockingEnable = ::ImGuiConfigFlags_DockingEnable;
     inline constexpr int Cond_FirstUseEver         = ::ImGuiCond_FirstUseEver;
+    // Typed as the underlying enum so they pass straight into
+    // DockBuilderSplitNode without the consumer naming ImGuiDir.
+    inline constexpr ::ImGuiDir Dir_Left  = ::ImGuiDir_Left;
+    inline constexpr ::ImGuiDir Dir_Right = ::ImGuiDir_Right;
+    inline constexpr ::ImGuiDir Dir_Up    = ::ImGuiDir_Up;
+    inline constexpr ::ImGuiDir Dir_Down  = ::ImGuiDir_Down;
 }
 #endif