Patch 1.10 Testing Next Week

[Flymo]

I think this is because people were able to macro 2 spells in one macro if the first was instant and didn't induce the global cooldown by using "SpellStopCasting()"

For example you could use PoM + Pyro in one button press or Nature's Swiftness + Healing Touch in one button press.  Not sure if you will still be able to with this change.
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I don't think this will stop you doing that. You can have any number of casts in a macro or add-on, it is just the cooldown that stops all but the first working. So using a trinket (which does not trigger global cooldown), SpellStopCasting(), then a cast should still work unless there's a further nerf in there somewhere.