When creating a local cell mask for _rendering_, the mask is tight bounding box around

the rendered cell plus 2px wide border (as the least safe extension). Thus, the target
mask image, into which the cell is finally rendered/copied, must have borders large
enough to host this small extra border -- which is usually the case as local mask of phase
functions are created with generous borders...
M    toolbox/bp_lists.cpp

ScmNewCellPositionCollide() functions do not let cell approach border closer than 2px.
This becomes essential when rendering the cell into the sceneMasks, as explained above.
M    cellScm.cpp