I do not know if this actually counts as a bug. Technically it causes no harm to resize the browser as the text does stay within focus (centered on the screen). But it is possible once you resize your browser to scroll all the way to the right into a large empty space.
Your User Agent is:
Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:49.0) Gecko/20100101 Firefox/49.0
Device Pixel Ratio:1
Screen Resolution:1920px x 1080px
Browser Window Size:1903 px x 971 px