Viewports: fixed unnecessary creation of temporary viewports when multiple docked windows got reassigned to a new node (created mid-frame) which already has a HostWindow

docs/CHANGELOG.txt

@@ -146,8 +146,10 @@ Docking+Viewports Branch:
 - IsWindowHovered: Fixed using ImGuiHoveredFlags_ChildWindows (without _RootWindow) from leaking the
   docking hierarchy. Added ImGuiHoveredFlags_DockHierarchy flag to consider docking hierarchy in the test.
 - Docking: fixed settings load issue when mouse wheeling. (#4310)
-- Drag and Drop: Fixed using BeginDragDropSource() or BeginDragDropTarget() inside a Begin() that
-  returned false because the window is docked. (#4515)
+- Docking + Drag and Drop: Fixed using BeginDragDropSource() or BeginDragDropTarget() inside a Begin()
+  that returned false because the window is docked. (#4515)
+- Viewports: Viewports: fixed unnecessary creation of temporary viewports when multiple docked windows
+  got reassigned to a new node (created mid-frame) which already has a HostWindow.
 
 
 -----------------------------------------------------------------------

imgui.cpp

@@ -12008,8 +12008,15 @@ static void ImGui::WindowSelectViewport(ImGuiWindow* window)
     else if ((flags & ImGuiWindowFlags_ChildWindow) || (flags & ImGuiWindowFlags_ChildMenu))
     {
         // Always inherit viewport from parent window
+        if (window->DockNode && window->DockNode->HostWindow)
+            IM_ASSERT(window->DockNode->HostWindow->Viewport == window->ParentWindow->Viewport);
         window->Viewport = window->ParentWindow->Viewport;
     }
+    else if (window->DockNode && window->DockNode->HostWindow)
+    {
+        // This covers the "always inherit viewport from parent window" case for when a window reattach to a node that was just created mid-frame
+        window->Viewport = window->DockNode->HostWindow->Viewport;
+    }
     else if (flags & ImGuiWindowFlags_Tooltip)
     {
         window->Viewport = g.MouseViewport;
@@ -12052,7 +12059,7 @@ static void ImGui::WindowSelectViewport(ImGuiWindow* window)
             else
                 window->ViewportAllowPlatformMonitorExtend = window->Viewport->PlatformMonitor;
         }
-        else if (window->Viewport && window != window->Viewport->Window && window->Viewport->Window && !(flags & ImGuiWindowFlags_ChildWindow))
+        else if (window->Viewport && window != window->Viewport->Window && window->Viewport->Window && !(flags & ImGuiWindowFlags_ChildWindow) && window->DockNode == NULL)
         {
             // When called from Begin() we don't have access to a proper version of the Hidden flag yet, so we replicate this code.
             const bool will_be_visible = (window->DockIsActive && !window->DockTabIsVisible) ? false : true;
@@ -13445,6 +13452,7 @@ static void ImGui::DockNodeRemoveWindow(ImGuiDockNode* node, ImGuiWindow* window
         if (node->HostWindow->ViewportOwned && node->IsRootNode())
         {
             // Transfer viewport back to the remaining loose window
+            IMGUI_DEBUG_LOG_VIEWPORT("Node %08X transfer Viewport %08X=>%08X for Window '%s'\n", node->ID, node->HostWindow->Viewport->ID, remaining_window->ID, remaining_window->Name);
             IM_ASSERT(node->HostWindow->Viewport->Window == node->HostWindow);
             node->HostWindow->Viewport->Window = remaining_window;
             node->HostWindow->Viewport->ID = remaining_window->ID;