This is a recurring problem for me:
I want to have two cells in a horizontal flow be spaced apart 15px.
If I add 15px of left margin to the cell on the right, then the cell on the right becomes shorter than the cell on the left.
I either have to add 8px of right margin on the left cell and 7px of left margin on the right cell, or use a spacer cell. Both of these are inconvenient.
One solution would be to have automatic layout margin, which would be extremely helpful in many other situations as well: https://getsatisfaction.com/antetype/topics/margin_between_content_cells
I think a single value would probably work, which as you said would function as the wrap margin as well.
Do we need seperate "layout margins" for x/y-axis, or is one value enough? For only one value, I guess this should be used as a spacer for wrapped lines/columns too? IIRC we have an older feature request were it was requested for tables, not sure if this is needed too.
Putting margin on every element in a container then applying negative padding is a good solution, but it's such a speed bump. Would absolutely love to see automatic margin in the next version.