Goal: get a 20/80 layout (a left sidebar and content of the right).
Create a view with two groups, give one a colspan of 2 and the second one a colspan of 8.
Observe that both groups take half the viewport, and tha their cell widths are set to 20% (correct) and 10% (not correct) respectively.
Issue is in addons/openerp/widgets/form/_form.py: max_width is computed as the maximum number of cells in a row of a table (in this case, 2), and then the width of a given cell (in colspans) is set to 1 if it's above max_width.
In this case, the first cell gets a colspan of 2 indeed (as 2 <= 2), but the second cell is above the limit and gets a colspan of 1 (instead of 8).
Patch attached computes max_length based on colspans (default: 1/cell if not set), doesn't seem to break any layout from a few checks but I'm not sure there aren't any views relying on the former behavior.
And it turns out that with that modification, max_length isn't very useful anymore I think (it shouldn't be possible for a colspan to be > max_length as colspans are computed from max_length. Unless there are negative colspans somewhere), so it could just as well be removed