In 3.x, you can do this by overriding the "_calculateClickTolerance" function of the map's popupManager property. For example:
map.popupManager._calculateClickTolerance = function(graphicsLayers) {
return 10;
};
That is a very simplistic example, but you could implement whatever logic you want inside that function as long as you return an integer (preferably non-negative).
I've read through Your Arcade Questions Answered but have not found a solution yet.
p.s.
In ArcGIS Desktop
under Selection -> Selection Options -> Selection tolerance
exists and wondering where the similar option is in AGOL (WAB) for configure popups.