There’s a horizontal scrollbar, when setting default gutenberg blocks to full width.
You might not see this on the mac, because the scrollbar is only visible, when you scroll.
But on Linux and Windows it’s visible and caused by
I had the same idea, the problem is, that all the padding calculations are wrong, since the width of the scrollbar messes up the whole calculation. And all the browsers have different scrollbar widths.
I have run into this while considering whether or not to use GP on a new site. When the scrollbar is visible, the expectation is that “full width” does not include the scrollbar, because that is what causes horizontal scroll. Regrettably, 100vw includes the scrollbar. To fix the horizontal scroll, the left and right margins for .no-sidebar .entry-content .alignfull need to be calc((-100vw + 15px) / 2 + 100% / 2), but only when the vertical scrollbar is visible.
Another way to solve this problem is to always show the scrollbar, even if it isn’t needed. But I wouldn’t suggest that.