if cell width is quite different, grid will show wrong cell when rotating

[Jia-Shen]

if portrait cell width is as twice as landscape, rotating the screen, grid will show wrong cell, this because the pageX is wrong. the contentsize is changed, the contentoffset is still the same value, so the pageX will be wrong. the contentoffset should be saved before the contentsize is changed. or keep the pagenumber all the time.