ImGui layout widgets filling width

ofApp.cpp

 if (ofxImGui::BeginWindow("TEST", mainSettings, flags)){
            ImGui::BeginGroup();
            {
                int numRows = 2;
                int numWidgets = 3;

                float spcx =ImGui::GetStyle().ItemSpacing.x;
                float spcy =ImGui::GetStyle().ItemSpacing.y;

                float maxWidth = ImGui::GetContentRegionAvail().x - spcx;
                float maxHeight = ImGui::GetContentRegionAvail().y - spcy;

                float mrw = maxWidth / numWidgets - spcx;
                float mrh = maxHeight / numRows - spcy;

                if (ImGui::Button("Button 1", ImVec2(mrw, mrh)))
                {}
                ImGui::SameLine();
                if (ImGui::Button("Button 2", ImVec2(mrw, mrh)))
                {}
                ImGui::SameLine();
                if (ImGui::Button("Button 3", ImVec2(mrw, mrh)))
                {}

                numWidgets = 8;
                mrw = maxWidth / numWidgets - spcx;
                
                for (int i = 0; i< numWidgets; i++)
                {
                    std::string n = "b"+ofToString(i);
                    if (ImGui::Button(n.c_str(), ImVec2(mrw, mrh)))
                    {}
                    if (i!=numWidgets-1)  ImGui::SameLine();
                }
            }
            ImGui::EndGroup();
        }
        ofxImGui::EndWindow(mainSettings);