Forechosen, I'm not 100% sure if your problem is what I think it is, so just let me know in case it isn't.
When you have a spell that can be up-casted, after you have clicked on the spell, a pop-up window appear, that lets you choose which level of spell slot to consume to cast the spell.
As far as I know, the spell cannot be "cancelled" at this point.
The two solutions I'm aware of are :
- Wait a moment. If you don't select a spell level, the pop-up window will disappear (and your command that you want to cast a spell will be dropped).
- Select a spell level. Then, when you are asked to select a target (including clicking anywhere, for spells without target), you can right-click to cancel.
Previously, the hotbar would have one icon for each spell-slot level of the spell (e.g. one icon for Bless level 1, one icon for Bless level 2). Larian introduced this new up-casting UI with Patch 3. Visibly, this was a quick and "ok the game doesn't crash" job, not a properly done job.
Given that this UI might not be the definitive one (and I certainly hope it won't, because it's still not good), I don't think Larian will bother polishing this.
