To check this behaviour, you need a Grid with a lot of records, set the paging to "All" records, or a lot of records and slide the scrollbar almost to the end
And then double click in a record to open the edit dialog. When it opens, it appear in the center of the screen, and the vertical scroolbar dissapears:
but as soon as i grab the dialog to drag it, the dialog disappears.
if you slide to the end of the grid, it will be there locked to the bottom.