Child window explicitly inherit collapse state from parent.

imgui.cpp

@@ -3045,8 +3045,10 @@ bool ImGui::Begin(const char* name, bool* p_opened, ImVec2 size, float fill_alph
     if (flags & ImGuiWindowFlags_ChildWindow)
     {
         IM_ASSERT((flags & ImGuiWindowFlags_NoTitleBar) != 0);
+        window->Collapsed = parent_window && parent_window->Collapsed;
+
         const ImVec4 clip_rect_t = window->ClipRectStack.back();
-        window->Collapsed = (clip_rect_t.x >= clip_rect_t.z || clip_rect_t.y >= clip_rect_t.w);
+        window->Collapsed |= (clip_rect_t.x >= clip_rect_t.z || clip_rect_t.y >= clip_rect_t.w);
 
         // We also hide the window from rendering because we've already added its border to the command list.
         // (we could perform the check earlier in the function but it is simpler at this point)