Backends: Win32: Added support for updating ParentViewportID after viewport creation.

This tends to be desirable for applicaton manipulating this value via ImGuiWindowClass::ParentViewportID.
I thought it would be a simple call to SetParent(), it's only slightly more complicated but added comments about it.

backends/imgui_impl_win32.cpp

@@ -950,11 +950,12 @@ void ImGui_ImplWin32_EnableAlphaCompositing(void* hwnd)
 struct ImGui_ImplWin32_ViewportData
 {
     HWND    Hwnd;
+    HWND    HwndParent;
     bool    HwndOwned;
     DWORD   DwStyle;
     DWORD   DwExStyle;
 
-    ImGui_ImplWin32_ViewportData() { Hwnd = nullptr; HwndOwned = false;  DwStyle = DwExStyle = 0; }
+    ImGui_ImplWin32_ViewportData() { Hwnd = HwndParent = nullptr; HwndOwned = false;  DwStyle = DwExStyle = 0; }
     ~ImGui_ImplWin32_ViewportData() { IM_ASSERT(Hwnd == nullptr); }
 };
 
@@ -974,6 +975,14 @@ static void ImGui_ImplWin32_GetWin32StyleFromViewportFlags(ImGuiViewportFlags fl
         *out_ex_style |= WS_EX_TOPMOST;
 }
 
+static HWND ImGui_ImplWin32_GetHwndFromViewportID(ImGuiID viewport_id)
+{
+    if (viewport_id != 0)
+        if (ImGuiViewport* viewport = ImGui::FindViewportByID(viewport_id))
+            return (HWND)viewport->PlatformHandle;
+    return nullptr;
+}
+
 static void ImGui_ImplWin32_CreateWindow(ImGuiViewport* viewport)
 {
     ImGui_ImplWin32_ViewportData* vd = IM_NEW(ImGui_ImplWin32_ViewportData)();
@@ -981,10 +990,7 @@ static void ImGui_ImplWin32_CreateWindow(ImGuiViewport* viewport)
 
     // Select style and parent window
     ImGui_ImplWin32_GetWin32StyleFromViewportFlags(viewport->Flags, &vd->DwStyle, &vd->DwExStyle);
-    HWND parent_window = nullptr;
-    if (viewport->ParentViewportId != 0)
-        if (ImGuiViewport* parent_viewport = ImGui::FindViewportByID(viewport->ParentViewportId))
-            parent_window = (HWND)parent_viewport->PlatformHandle;
+    vd->HwndParent = ImGui_ImplWin32_GetHwndFromViewportID(viewport->ParentViewportId);
 
     // Create window
     RECT rect = { (LONG)viewport->Pos.x, (LONG)viewport->Pos.y, (LONG)(viewport->Pos.x + viewport->Size.x), (LONG)(viewport->Pos.y + viewport->Size.y) };
@@ -992,7 +998,7 @@ static void ImGui_ImplWin32_CreateWindow(ImGuiViewport* viewport)
     vd->Hwnd = ::CreateWindowEx(
         vd->DwExStyle, _T("ImGui Platform"), _T("Untitled"), vd->DwStyle,       // Style, class name, window name
         rect.left, rect.top, rect.right - rect.left, rect.bottom - rect.top,    // Window area
-        parent_window, nullptr, ::GetModuleHandle(nullptr), nullptr);           // Parent window, Menu, Instance, Param
+        vd->HwndParent, nullptr, ::GetModuleHandle(nullptr), nullptr);          // Owner window, Menu, Instance, Param
     vd->HwndOwned = true;
     viewport->PlatformRequestResize = false;
     viewport->PlatformHandle = viewport->PlatformHandleRaw = vd->Hwnd;
@@ -1029,10 +1035,26 @@ static void ImGui_ImplWin32_ShowWindow(ImGuiViewport* viewport)
 
 static void ImGui_ImplWin32_UpdateWindow(ImGuiViewport* viewport)
 {
-    // (Optional) Update Win32 style if it changed _after_ creation.
-    // Generally they won't change unless configuration flags are changed, but advanced uses (such as manually rewriting viewport flags) make this useful.
     ImGui_ImplWin32_ViewportData* vd = (ImGui_ImplWin32_ViewportData*)viewport->PlatformUserData;
     IM_ASSERT(vd->Hwnd != 0);
+
+    // Update Win32 parent if it changed _after_ creation
+    // Unlike style settings derived from configuration flags, this is more likely to change for advanced apps that are manipulating ParentViewportID manually.
+    HWND new_parent = ImGui_ImplWin32_GetHwndFromViewportID(viewport->ParentViewportId);
+    if (new_parent != vd->HwndParent)
+    {
+        // Win32 windows can either have a "Parent" (for WS_CHILD window) or an "Owner" (which among other thing keeps window above its owner).
+        // Our Dear Imgui-side concept of parenting only mostly care about what Win32 call "Owner".
+        // The parent parameter of CreateWindowEx() sets up Parent OR Owner depending on WS_CHILD flag. In our case an Owner as we never use WS_CHILD.
+        // Calling ::SetParent() here would be incorrect: it will create a full child relation, alter coordinate system and clipping.
+        // Calling ::SetWindowLongPtr() with GWLP_HWNDPARENT seems correct although poorly documented.
+        // https://devblogs.microsoft.com/oldnewthing/20100315-00/?p=14613
+        vd->HwndParent = new_parent;
+        ::SetWindowLongPtr(vd->Hwnd, GWLP_HWNDPARENT, (LONG_PTR)vd->HwndParent);
+    }
+
+    // (Optional) Update Win32 style if it changed _after_ creation.
+    // Generally they won't change unless configuration flags are changed, but advanced uses (such as manually rewriting viewport flags) make this useful.
     DWORD new_style;
     DWORD new_ex_style;
     ImGui_ImplWin32_GetWin32StyleFromViewportFlags(viewport->Flags, &new_style, &new_ex_style);

docs/CHANGELOG.txt

@@ -114,6 +114,7 @@ Docking+Viewports Branch:
   keeping ID alive when calling low-level ButtonBehavior() directly). (#5181, #2645)
 - Docking: [Internal] DockBuilderDockWindow() API calls don't clear docking order
   if the target node is same as existing one.
+- Backends: Win32: Added support for changing ParentViewportID after viewport creation.
 
 
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