Docking: Fixed dock node with single window accidental stealing of non-owned host viewport (leading to general havoc).

imgui.cpp

@@ -7381,6 +7381,7 @@ static void ImGui::UpdateViewports()
     // Update main viewport with current platform position and size
     ImGuiViewportP* main_viewport = g.Viewports[0];
     IM_ASSERT(main_viewport->ID == IMGUI_VIEWPORT_DEFAULT_ID);
+    IM_ASSERT(main_viewport->Window == NULL);
     ImVec2 main_viewport_platform_pos = ImVec2(0.0f, 0.0f);
     if ((g.IO.ConfigFlags & ImGuiConfigFlags_ViewportsEnable))
         main_viewport_platform_pos = g.PlatformIO.Platform_GetWindowPos(main_viewport);
@@ -10524,7 +10525,11 @@ static void ImGui::DockNodeUpdate(ImGuiDockNode* node)
         {
             single_window->Viewport = node->HostWindow->Viewport;
             single_window->ViewportId = node->HostWindow->ViewportId;
-            single_window->Viewport->Window = single_window;
+            if (node->HostWindow->ViewportOwned)
+            {
+                single_window->Viewport->Window = single_window;
+                single_window->ViewportOwned = true;
+            }
         }
 
         DockNodeHideHostWindow(node);

imgui_internal.h

@@ -657,7 +657,7 @@ struct ImGuiViewportP : public ImGuiViewport
     float               Alpha;                    // Window opacity (when dragging dockable windows/viewports we make them transparent)
     float               LastAlpha;
     int                 PlatformMonitor;
-    ImGuiWindow*        Window;
+    ImGuiWindow*        Window;                   // Set when the viewport is owned by a window
     ImDrawList*         OverlayDrawList;          // For convenience, a draw list we can render to that's always rendered last (we use it to draw software mouse cursor when io.MouseDrawCursor is set)
     ImDrawData          DrawDataP;
     ImDrawDataBuilder   DrawDataBuilder;