ui-grid not show all data until scroll

i have ui-grid in popup window with "ui-grid-auto-resize" and it bind to 30 row in initial loading and grid default height is "300px" it show 14 row and when i scroll to the last row it seem that it loading the remain data on scrolling. i have post action on controller when i bind the data on it grid data is equal to "null" if only i scroll the grid and set to the first 15 rows if i not scroll the grid. when i watch the formcollection of form form read last 15 rows of data when grid scrolling with index begin with 15. i set the height of grid to amount of row using javascript and it work fine and grid data bind to control action correct. but i need scroll inside the gird because it's inside popup window and height of grid exceed window size