It is due to scaling. Say the client runs with the native resolution of the display (eg 1920x1080). When it runs in window mode, it can't display the whole height of 1080 pixel, due to the frame decorations and/or taskbar. In order to display the whole window, it has to resize the content. Due to it only being a few lines of pixel to short, it has to omit a line every now and then. That is the abberation you see. (In a nutshell).
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