Created attachment 217481 [details]
ShapeOutsideInfo clamps top margin to 0 when calculating the line's offset relative to the float.
Created attachment 225077 [details]
Updated test case using proper reference box
The original test case assumes that the shape is sized and positioned based on the content box, so update it to explicitly use the content-box sizing.
As far as I can tell, this has been fixed. I believe my fix for bug 128631 is what fixed it.