If you want to keep the popup edit form of a RadGrid in a static place so it denotes to the user which item they are editing (in addition to the row highlight that the grid provides), you can use the approach below.
This can also solve some problems where the form may move especially if postbacks inside it occur and/or scrollbars get lost due to partial page rendering removing some scrollbars during parsing of the new content, and thus changes the position of the grid and the popup form.
The gist is that you can make the table row that contains the popup form have position:relative, and use the OnPopupShowing event to move the form to its beginning (or another location as per your preferences):
There are two other approaches you can consider: