Incorrect size/position when background-size is 'contain'

[Leftium]

Live demo: https://ripples-bug.glitch.me/ (Resize so window is much wider than tall)

Expected: Ripples canvas should be square
Actual: Ripples canvas is rectangular

If Ripples is disabled, the background image is square, and rendered as a square as a result of background-size:contain. However, the Ripples canvas is sized to the full geometry of its parent element.

(Is this the "TODO: background-clip" from the source?)

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I quickly hacked my own version of Ripples that forces the canvas to be a correctly positioned square during initialization and updateSize(), but it would be nice to have a general fix.