Some basic problems to study. The size of the browser. Is it wider at some point than another?
Obviously, in one browser, but just in case, I would test it in several browsers because you want your code to be cross-compatible.
To me, it looks like a width issue.
Perhaps you have a div #login or some strange named in the upper right corner that contains this content and floats it, and also that the content does not respond, but the div itself. When the div shrinks a bit, the content itself does not delete or release the rest of the content.
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