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Test hi-dpi "retina" display, where there are four times as many pixels as the window's (height x width) would have you believe. On hi-res displays, the "HighRes" window will have one-pixel-wide vertical black bars within the red-to-purple gradient, while in the "LowRes" window they'll be two-pixel-wide. On regular displays, LowRes will have the one-pixel bars, and HiRes should be a down-sampled gradient with no bars.