Edge effects in developers tend to disappear when agitation is constant. To try the ultimate edge effect look into the technique of stand developing. The edge effects happen from fresher developer diffusing past a low density area into a higher density area and being fresher than the stuff already there due to less used up where it came from it causes an increase in density along that border in the higher density area. So you can see that agitation reduces the opportunity for this to happen.
With stand developing a highly diluted developer like Rodinal is used and the times extended up to and beyond an hour, agitating only at the outset to disrupt any bubbles clinging to the film. Some films don't work well with this, streaking from sprocket holes is a common malady.