ok, I'm taking at look at disp.box and the relevant images...

It looks to me like pagedown.bmp doesn't stay on top of dispbgx.bmp, which means that by clicking it, you're actually clicking the background, if you understand what I mean. Maybe it would work if you make the triangle-area of dispbgx.bmp magic pink. I'm not sure, but I think that this will help in keeping pagedown.bmp "on top" so to speak.

ps. by looking at the tooltip while hovering over the triangle, you'll easily find out whether it is actually showing through or not.