Your feedback is quite important, that helps us to improve the grid with every version.
I understand that you want to use scrolling only for the outer grid, no scrolling for inner grids and your inner nested grids may exceed the view port of outer one.
In this regard non virtual mode would be fixed w.r.t the discussed issues and I guess that's what you need.
As far as virtual mode is concerned, it works the way as designed. It scrolls by row and column units similar to Excel. There is no immediate plan to alter it. In virtual mode, nesting is only useful either by using scrolling or paging for inner grids so that they don't exceed the view port of outer grid.