Right now, Fluxer has optimistic messages (the message appears in the channel grayed out until the server confirms that it has actually been sent). So what happens is, the optimistic message is sent, then when the server confirms the success, the message is recreated entirely and replaces the optimistic variant. This makes the task much harder than "a line of script". Trust me, I tried with custom CSS lol.
So Fluxer would need to change the way messages appear in the channel for animations to work. Maybe we could try to make sure that, instead of deleting the optimistic variant and recreating the confirmed message, the message's root element's content would be edited so that the root element itself could be animated properly? I'm sure someone could try this out.
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Comment by Speykious