You have selected to show a window without its border. Without the border, you will not be able to enable the border again using the mouse: use the window operations menu instead, activated using the %1 keyboard shortcut.
Jo ha opdracht jûn om in finster sûnder râne wer te jaan. Sûnder râne kinne jo it finstermenu net mear mei de mûs oanroppe. Jo kinne it finstermenu wol oproppe mei de fluchtoets% 1.