* EDIT
Using CreatePanel with a PANEL_BORDER style and the smallest height (4 pixels) produces:

Anything smaller and the panel disappears.
However, this seems to look about right:

Using a panel with a PANEL_GROUP style and the following values:
Using CreatePanel with a PANEL_BORDER style and the smallest height (4 pixels) produces:

Anything smaller and the panel disappears.
However, this seems to look about right:

Using a panel with a PANEL_GROUP style and the following values:
win:TGadget = CreateWindow("separator - panel",10,10,180,90,Null,WINDOW_TITLEBAR) CreatePanel -2,20,win.ClientWidth()+2,8,win,PANEL_GROUP Repeat WaitEvent Until EventID()=EVENT_WINDOWCLOSE EndNote the -2 x offset. This is needed to remove an odd looking white pixel from the start of the line separator. Even though the line is obviously too close to the edge now. I know .. I'm hard to please.