A nasty one!

 

To reproduce, go to Horizontal scroll mode and create an all-day event on, say, 28 Feb 2020. Edit the event, make it last to, say, 28 March 2020. For me, this already causes IQ to slow down, but that may be subjective. After the change, scroll around a bit, again, for me it's quite sluggish. Re-open the event details dialog and try to change item duration back to one day, so set the second date for the All-day event to 28 Feb 2020 also and okay / save. IQ refuses to do this, the item will keep its duration (try scrolling). Re-opening the event details dialog will confirm the change has failed. To fix, I could only delete the item or change the starting date to 27 Feb 2020. As this didn't happen when I tried the same for Feb 2019, I'm guessing it has something to do with the leap day?

 

Many thanks!