With my mobile Sony Android 720x1280 pixels, the mobile variant DOES load centrally, but one can slide the page left and back to centre' on page load and from the menu button.

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When the share buttons come into view, I can no longer slide the page left and it appears as expected.

On your desktop version the share buttons are to the left of the page, and on the mobile version across the bottom of the page. However, something appears strange in the transition from one to the other in that the share buttons move to the bottom of the page before the mobile variant kicks in.

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I am not sure if that is what you would expect, but if you delete the share buttons from both the desktop and the mobile variant does the problem still persist? Seems to me that the share buttons are not resizing until they come into view on the mobile variant and that prior to that, they are extending beyond the page width boundary

This is really guesswork, but maybe it helps?

Gary