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.
Heu triat mostrar una finestra sense la vora. Sense la vora no podreu tornar- la a habilitar usant el ratolí: per a fer- ho useu el menú d' operacions de finestra, activat usant la drecera de teclat% 1.