Begin: Simplified code and fixed a bug where appending into a window a second time (from a remote window) would incorrectly overwrite RootWindow with the current window in the stack. Our docking code uses this pattern.

imgui.cpp

@@ -4181,21 +4181,17 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
         g.SetNextWindowFocus = false;
     }
 
-    // Update known root window (if we are a child window, otherwise window == window->RootWindow)
-    int root_idx, root_non_popup_idx;
-    for (root_idx = g.CurrentWindowStack.Size - 1; root_idx > 0; root_idx--)
-        if (!(g.CurrentWindowStack[root_idx]->Flags & ImGuiWindowFlags_ChildWindow))
-            break;
-    for (root_non_popup_idx = root_idx; root_non_popup_idx > 0; root_non_popup_idx--)
-        if (!(g.CurrentWindowStack[root_non_popup_idx]->Flags & (ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_Popup)) || (g.CurrentWindowStack[root_non_popup_idx]->Flags & ImGuiWindowFlags_Modal))
-            break;
-    window->ParentWindow = parent_window;
-    window->RootWindow = g.CurrentWindowStack[root_idx];
-    window->RootNonPopupWindow = g.CurrentWindowStack[root_non_popup_idx];      // Used to display TitleBgActive color and for selecting which window to use for NavWindowing
-
     // When reusing window again multiple times a frame, just append content (don't need to setup again)
     if (first_begin_of_the_frame)
     {
+        // Initialize
+        window->ParentWindow = parent_window;
+        window->RootWindow = !(flags & ImGuiWindowFlags_ChildWindow) ? window : parent_window->RootWindow;
+        window->RootNonPopupWindow = !(flags & (ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_Popup)) || (flags & ImGuiWindowFlags_Modal) ? window : parent_window->RootNonPopupWindow; // Used to display TitleBgActive color and for selecting which window to use for NavWindowing
+        //window->RootNavWindow = window;
+        //while (window->RootNavWindow->Flags & ImGuiWindowFlags_NavFlattened)
+        //    window->RootNavWindow = window->RootNavWindow->ParentWindow;
+
         window->Active = true;
         window->OrderWithinParent = 0;
         window->BeginCount = 0;