ImGui::GetStyle().WindowRounding = 0.0f;// <- Set this on init or use ImGui::PushStyleVar()
ImGui::GetStyle().ChildRounding = 0.0f;
ImGui::GetStyle().FrameRounding = 0.0f;
ImGui::GetStyle().GrabRounding = 0.0f;
ImGui::GetStyle().PopupRounding = 0.0f;
ImGui::GetStyle().ScrollbarRounding = 0.0f;