Docking: Fix appending to windows causing a child window to be insert…

…ed to g.WindowsFocusOrder which eventually causes a crash. (ocornut#5515)

imgui.cpp

@@ -6087,7 +6087,10 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     const bool window_just_created = (window == NULL);
     if (window_just_created)
         window = CreateNewWindow(name, flags);
-    else
+
+    const int current_frame = g.FrameCount;
+    const bool first_begin_of_the_frame = (window->LastFrameActive != current_frame);
+    if (first_begin_of_the_frame)
         UpdateWindowInFocusOrderList(window, window_just_created, flags);
 
     // Automatically disable manual moving/resizing when NoInputs is set
@@ -6097,8 +6100,6 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     if (flags & ImGuiWindowFlags_NavFlattened)
         IM_ASSERT(flags & ImGuiWindowFlags_ChildWindow);
 
-    const int current_frame = g.FrameCount;
-    const bool first_begin_of_the_frame = (window->LastFrameActive != current_frame);
     window->IsFallbackWindow = (g.CurrentWindowStack.Size == 0 && g.WithinFrameScopeWithImplicitWindow);
 
     // Update the Appearing flag