Ok, I'll bite... How? A state flat tax will tax this new raise at the same flat rate as all the existing pay. The marginal federal tax will tax the new raise,

Ok, I'll bite... How? A state flat tax will tax this new raise at the same flat rate as all the existing pay. The marginal federal tax will tax the new raise, in your scenario, at the new, higher tax bracket. But your scenario would have the gov't taxing this new cash into the red, reducing her take home pay. That's not gonna happen. Period.