feat: viewports feature — panels detach into real OS windows

New independent `viewports` feature (usually combined with docking):
- core exports GetMainViewport/UpdatePlatformWindows/
  RenderPlatformWindowsDefault/SetNextWindowPos + ConfigFlags_ViewportsEnable
  (gated by MCPP_FEATURE_VIEWPORTS).
- imgui.app facade enables ViewportsEnable and renders platform windows each
  frame (with GL context backup/restore); GlfwOpenGL3 gains GetCurrentContext.
- docking example requests ["docking", "viewports"] and spawns a 'Detached'
  panel outside the main window — verified: a separate OS window appears in
  the X window tree and renders (drag back inside to re-dock).
Default (feature-off) build/tests unchanged.

Author: d2learn-org

examples/docking/mcpp.toml

@@ -5,4 +5,4 @@ description = "Dockable-windows demo using the imgui `docking` feature"
 license     = "MIT"
 
 [dependencies]
-imgui = { path = "../..", features = ["docking"] }
+imgui = { path = "../..", features = ["docking", "viewports"] }

examples/docking/src/main.cpp

@@ -1,9 +1,11 @@
-// Docking demo — `imgui = { ..., features = ["docking"] }`.
+// Docking + multi-viewport demo —
+//   imgui = { ..., features = ["docking", "viewports"] }
 //
 // 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.
+// a real dock node: drag any tab to re-split, stack, or float it.
+// With `viewports`, drag a tab OUTSIDE the main window and it detaches into
+// its own OS window; the "Detached" panel below starts outside on purpose.
 import imgui.core;
 import imgui.app;
 
@@ -45,5 +47,16 @@ int main() {
         ImGui::Begin("Console");
         ImGui::TextUnformatted("Log pane (bottom split).");
         ImGui::End();
+
+        // Starts OUTSIDE the main window: with `viewports` this is a real,
+        // separate OS window from the very first frame.
+        const auto* main_vp = ImGui::GetMainViewport();
+        ImGui::SetNextWindowPos(
+            ImVec2{main_vp->Pos.x + main_vp->Size.x + 60.0f, main_vp->Pos.y + 80.0f},
+            ImGui::Cond_FirstUseEver);
+        ImGui::Begin("Detached");
+        ImGui::TextUnformatted("I live in my own OS window.");
+        ImGui::TextUnformatted("Drag me back inside to re-dock.");
+        ImGui::End();
     });
 }

mcpp.toml

@@ -27,6 +27,9 @@ default = []
 # docking: export the Dock* API surface and auto-enable
 # ImGuiConfigFlags_DockingEnable in the imgui.app facade.
 docking = []
+# viewports: ImGui windows dragged outside the main window detach into real
+# OS windows (multi-viewport). Usually combined with docking.
+viewports = []
 
 [dependencies]
 compat.imgui = "1.92.8-docking"

src/app.cppm

@@ -57,6 +57,11 @@ export namespace ImGui::App {
         // `docking` feature: enable dockable windows out of the box.
         ImGui::GetIO().ConfigFlags |= ImGui::ConfigFlags_DockingEnable;
 #endif
+#ifdef MCPP_FEATURE_VIEWPORTS
+        // `viewports` feature: windows dragged outside the main window
+        // detach into real OS windows.
+        ImGui::GetIO().ConfigFlags |= ImGui::ConfigFlags_ViewportsEnable;
+#endif
 
         if (!Backend::Init(window)) {
             const auto error = Backend::LastError();
@@ -84,6 +89,15 @@ export namespace ImGui::App {
             Backend::ClearColor(0.10f, 0.10f, 0.12f, 1.0f);
             Backend::ClearColorBuffer();
             Backend::RenderDrawData(ImGui::GetDrawData());
+#ifdef MCPP_FEATURE_VIEWPORTS
+            // Render the detached OS windows, then restore the main context.
+            {
+                auto* backup = Backend::GetCurrentContext();
+                ImGui::UpdatePlatformWindows();
+                ImGui::RenderPlatformWindowsDefault();
+                Backend::MakeContextCurrent(backup);
+            }
+#endif
             Backend::SwapBuffers(window);
         }
 

src/backends/glfw_opengl3.cppm

@@ -62,6 +62,10 @@ export namespace ImGui::Backend {
             GlfwPlatform::MakeContextCurrent(window);
         }
 
+        static Window* GetCurrentContext() {
+            return GlfwPlatform::GetCurrentContext();
+        }
+
         static void SwapInterval(int interval) {
             GlfwPlatform::SwapInterval(interval);
         }

src/backends/platform_glfw.cppm

@@ -71,6 +71,10 @@ export namespace ImGui::Backend {
             glfwMakeContextCurrent(window);
         }
 
+        static Window* GetCurrentContext() {
+            return glfwGetCurrentContext();
+        }
+
         static FbSize FramebufferSize(Window* window) {
             int width = 0;
             int height = 0;

src/core.cppm

@@ -65,3 +65,18 @@ export namespace ImGui {
     inline constexpr ::ImGuiDir Dir_Down  = ::ImGuiDir_Down;
 }
 #endif
+
+// Multi-viewport surface — gated by the `viewports` feature: ImGui windows
+// dragged (or positioned) outside the main window become real OS windows.
+#ifdef MCPP_FEATURE_VIEWPORTS
+export using ImGuiViewport = ::ImGuiViewport;
+
+export namespace ImGui {
+    using ::ImGui::GetMainViewport;
+    using ::ImGui::UpdatePlatformWindows;
+    using ::ImGui::RenderPlatformWindowsDefault;
+    using ::ImGui::SetNextWindowPos;
+
+    inline constexpr int ConfigFlags_ViewportsEnable = ::ImGuiConfigFlags_ViewportsEnable;
+}
+#endif