The only time I have ever clicked on this has been by accident clicking on blank space only to have the pop up appear afterwards and the intention of my click being assigned to the pop up, rather than simply bringing that stacks cards into view on the right side of the screen. This sends me into a study session which I don't want to be in and have to exit, but more importantly also creates an ongoing study session which will have to be resolved in the next sync with whatever ongoing session I may have on my iPhone.

Perhaps you can change this so a double click is required on the pop up, or if possible, make it so the ability for the pop up to respond to a click is delayed for a second after the pop up appears. I'm guessing it is not possible for the pop up to be moved so it spills over to the right on to the next panel, which if it were possible, would also help. Or you could just get rid of it altogether, because I would be that number of people that make use of the pop up to study due notes stack by stack are fewer than the number that are regularly inconvenienced by having their click assigned to a pop up rather than the blank space they intended to click on.

ps Or perhaps make it so that you have to click on the number due for the pop up to appear, rather than just hover the mouse over an area surrounding the number due (which would require two clicks to activate the study session)