@mitranasiri if you open opera settings and type "shortcuts" into search box top right, in the results should be "Configure shortcuts". under that list that opens when you click configure shortcuts will be "toggle full screen". set a key bind or look what key is set there and try it. opera should toggle between full screen (i.e. hide the taskbar etc and show it full screen) or just maximized with taskbar showing. Try it and see what happens... it is either normal behaviour or you have some issue, but may help to drill it down a little, I think i am stating the obvious and you probably know this stuff better than me...
