Choose Format - Arrange - Send to Back (LibreOfficeDev Writer, LibreOfficeDev Calc).
Choose Shape - Arrange - Send to Back (LibreOfficeDev Draw).
(Shift) + (Command)(Ctrl) + (-) 記号 (LibreOfficeDev Impress、LibreOfficeDev Draw)
Open context menu - choose Arrange - Send to Back (LibreOfficeDev Impress).