If none of these other suggestions I have gotten a solution that may work. Try to drag the steam app more inward. This problem randomly occurs to people such as myself and a few others whereupon launching the steam app will automatically drag the windows borders to the edges of your screen looking as if you were in full screen mode. The easiest way to tell if this has happened to you is to press the fullscreen button on the top right of your screen. If the window does not look like it has been affected then you may possibly be encountering this bug.

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