A method (of generating a layout diagram) includes: identifying, in the layout diagram, a group of three or more cells which violates a horizontal constraint vector (HCV) and is arranged so as to exhibit two or more vertically-aligned edge-pairs (VEPs); each VEP including two members representing at least partial portions of vertical edges of corresponding cells of the group; relative to a horizontal direction, the members of each VEP being disposed in edgewise-abutment and separated by a corresponding actual gap; and the HCV having separation thresholds, each of which has a corresponding VEP and represents a corresponding minimum gap in the horizontal direction between the members of the corresponding VEP; and for each of at least one but fewer than all of the separation thresholds, selectively moving a given one of cells corresponding to one of the members of the corresponding VEP thereby to avoid violating the HCV.