(1) "in fullscreen mode" (2) "in full-screen mode" and (3) "in full screen mode" are possible in all form of modern English. Whilst 'fullscreen' as a single word has not yet reached the pages of most dictionaries, there is no doubt that it soon will, as 'widescreen' has already done. This company will popularise the single word usage.
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(All three forms) Rover_KEare possible in all form of modern English.I see. Thank you Rover!