When using block selection on a grid, it will automatically scroll upward, always. This problem only seems to occur when there are multiple grids on the screen. If I click on a different grid, then do a block selection on the original grid, it will scroll to the top of the grid every time the mouse is moved.
Unfortunately we cannot reproduce this issue locally. What version of dhtmlxGrid do you use? Please provide us example where we can reproduce it. (You can send such example directly to the support@dhtmlx.com)