I don't think it can be done without editing the GDL script.
Your workaround is to use the fill tool
(like you did in your screen shot?).
Measure the radius of the detail box's corner and
copy/paste the distance into the "Fillet/Chamfer" command.
It's a quick/easy way to get a rectangular fill to match the detail bubble.
Marc Corney, Architect
Red Canoe Architecture, P. A.
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ArchiCAD 25 (5010 USA Full) //// ArchiCAD 27 (4030 USA Full)