Source--Metalab Slack Design History
Ref: https://www.metalab.co/projects/slack
We strived to fill the product with delightful, and unexpected, interactions—the logo animates in a burst of color as it loads, the modals slide down from the top of the screen, gifs appear at random like Russian roulette
You can see Slack won because it was fun. Also they clearly mention Slack's GIFs which is often problematic due to it's Slack's broken hierarchy of priority.
I don't think that "fun" is the problem, but I do think Slack takes it a little too far sometimes.
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