If I understand what you are trying to achieve here then in your LayoutControl handler make the following change:
-- put the rect of btn "btnCheck" of me into theFieldRect
-- put item 3 of pControlRect - 5 into item 3 of theFieldRect
-- set the rect of btn "btnCheck" of me to theFieldRect
set the left of btn "btnCheck" of me to the right of fld "cat" of me
However I think the resizing of the Cat field is wrong too. Try something like:
on LayoutControl pControlRect
set the right of btn "btnCheck" of me to item 3 of pControlRect-4
set the right of fld "Cat" of me to the left of btn "btnCheck" of me
get the rect of fld "Line" of me
put the left of fld "Cat" of me into item 3 of it
set the rect of fld "Line" of me to it
put the formattedHeight of fld "Line" of me + item 2 of it into item 4 of it
set the rect of fld "Line" of me to it
put item 4 of it into item 4 of pControlRect
set the rect of graphic "Background" of me to pControlRect
end LayoutControl
To edit the LayoutControl script you need to open the datagrid property inspector and click on the Row Behavior... button. This will present the script editor for the behavior of the row template.