Yessir, you're absolutely right, but now I have another weird problem.
Now the game thinks I'm pushing f10 all the time... I used the line if (controller(left_click)) {....}, and I also tried using the controller number. My Logic.91 code is as such: set.key(0, 68, left_click); //clicking on item in interface screen.
Yet again, I made sure to check the help docs before I asked, but it made no mention.
AGISCI, you're my hero!