What do you think about moving this constant to application-data where the configuration of the homescreen app is available? [1]

and in the init method [2] recover this value:

PANNING_LOOKAHEAD = Configurator.getSection('panning_lookahead');

[1] https://github.com/mozilla-b2g/gaia/blob/master/build/applications-data.js#L126
[2] https://github.com/mozilla-b2g/gaia/blob/master/apps/homescreen/js/grid.js#L923