The animation that you are viewing was constructed so that the area of the elementary volume of sandstone was retained throughout the experiment. Pay close attention to the colored material lines. Do the lengths of some material lines change? Do some lines rotate during pure shear?
As you watch the animation of plane pure shear note that material lines that are parallel to the Y axis become shorter as pure shear progresses while material lines parallel to X are extended. In contrast, the lengths of material lines oblique to both X and Y lengthen during pure shear and rotate toward the X axis.