It might be because I built my computer in 2007, DDR2 and 3GHz Dual Core, which seems powerful enough but the underlying technology is inferior to today's CPUs, so I'd have a bigger window to do this in. But during the opponents turn I can still interact with the interface. It is still quite choppy.
The easiest way to replicate it would be to cast a strategic offensive spell. End turn then quickly open up the spell book and cast something like star fall or tremor on an enemy unit/MnR. Not sure how easy it is to replicate, since I never really do it on purpose, more of in a panicked, 'OH CRAP! CLICK THINGS QUICKLY!', state. I think you can even equip or change research as well (pretty sure a traded a horse in my Gilden game but by the time I clicked trade and equip it might have been my turn.)
Sometimes doing things when you're not supposed to causes a problem. I look at the stat screen for units in tactical battle while the AI is taking its move. Which occasionally locks up the battle. Clicking auto battle until the turn progresses then quickly taking it off should unlock it, even without your unit taking action.
