This one puzzles me... if I have a grid, let's say 5 cols by 5 rows.
Now I put a 3-col stacklayout (call it SL1) at col=1 and colwidth=3. This centers SL1 horizontally, leaving 1 blank col on either side.
Inside SL1, I have another stacklayout (SL2). If I put specify grid position for SL2, is it relative to SL1 or absolute to the overall grid structure? In other words, is col 1 the middle column of SL2 or just the 2nd column of the 5 in overall grid?