As per @Tgr report:

In T238460#5691928, @Tgr wrote:

When the modal dialog opens, the whole page can shift left. (Only seems to happen on narrow mobile screens; I saw it in desktop Chrome in mobile emulation mode using iPhone 8.) That's due to rGOJU6a5914dda8f8: Compensate for loss of margin when opening modals and not related to the patch, but it becomes much more jarring now that it affects the button that's being clicked. Not sure what to do about that; it seems like an OOUI bug, but maybe I just don't understand what it is for (or maybe it doesn't affect real mobile devices?).

before click	after click

It does affect real mobile devices - I've been observing it on iPhone 6s, where it looks like this:

Before click	After click

Note:The issue of the article card not fitting the screen (the navigation arrows are not quite visible) should be fixed after T238280 is resolved.