When a shoutbox refreshes, on my DSi it may take multiple screen refreshes/redraws before all shouts are visible again. Hardware limitation, really.
What happens as a result is that when sending messages to a shoutbox, the least recent shout pops up to the top for a brief moment, which is unexpected and undesired. Probably because shouts are loaded from least recent to most recent, or from the bottom up.
If, instead, shouts are loaded from most recent to least recent, or from top down, the order of shouts would be preserved even if load times are nonzero.
TL;DR: reverse the direction of shouts loading from bottom up to top down for a better user experience.
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