Perhaps I'm missing something but is there a way to negate the interruptible checkbox in the "Spell Cast" condition so you could, for example, show something only when your target is casting a non-interruptible spell?

So I managed to do this by checking a Condition Icon, but it would still be nice to have a checkbox directly in the "Spell Cast" condition.

That's a good workaround. I'll leave this open to gauge interest, but in 14 years of TellMeWhen nobody else has requested this so I wouldn't hold your breath on it getting prioritized as a feature.

Have suggested this as a workaround on the TMW discord ~2 times as well if memory serves.