Begin: Fixed appending into a child window with a second Begin() from a different window stack querying the wrong window for the window->Collapsed test.

imgui.cpp

@@ -4106,12 +4106,14 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
     else
         flags = window->Flags;
 
+    // Parent window is latched only on the first call to Begin() of the frame, so further append-calls can be done from a different window stack
+    ImGuiWindow* parent_window = first_begin_of_the_frame ? (!g.CurrentWindowStack.empty() ? g.CurrentWindowStack.back() : NULL) : window->ParentWindow;
+    IM_ASSERT(parent_window != NULL || !(flags & ImGuiWindowFlags_ChildWindow));
+
     // Add to stack
-    ImGuiWindow* parent_window = !g.CurrentWindowStack.empty() ? g.CurrentWindowStack.back() : NULL;
     g.CurrentWindowStack.push_back(window);
     SetCurrentWindow(window);
     CheckStacksSize(window, true);
-    IM_ASSERT(parent_window != NULL || !(flags & ImGuiWindowFlags_ChildWindow));
 
     bool window_just_activated_by_user = (window->LastFrameActive < current_frame - 1);   // Not using !WasActive because the implicit "Debug" window would always toggle off->on
     if (flags & ImGuiWindowFlags_Popup)