Interesting, let's see if I can help figure this out. I'm afraid it may not be a bug but your phone. Let me first just say you shouldn't have to "double click" if you're using a phone. In the settings there is a Touch or Mouse function, you want to be using the Touch function ( you probably already know this, I'm just trying to cover all the problems)
If you're HAVE to use the mouse setting over the touch setting, your double clicks have to be PRECISE. I will agree the buttons are a little small and close to the edge. I'll try my best to fix that in an update but I can't promise it's going to change a lot from their current position. With it being so close to the edge of the screen, I assume you're having problems with the home button back button on your phone. If this is the case, the only thing I could really suggest right now is flipping your phone over that way the buttons are on the top of your phone.
!! (Please test this for me) Your having problems with the arrow buttons that control how many cards you see Plus controls viewing the selection cards. The arrow button stays tan when it's inactive or untouched, but with the mouse if you were to hover over the arrow button it would turn white (this is usually the first click out of the double click I ask phone users to do) if it didn't turn white, then you're not hovering over it. If it's white (currently hovering over it) clicking on it would make it turn brown ( this would be your precise second click).
So just to summarize, get the arrow button to turn white, if you're able to do that, the only problem is precision ( if this is for sure not working, then we may have a bigger problem to figure out) . Again, if you're using a phone I heavily suggest you use the touch settings in the menu. Lastly whatever browser you may be using, try to use the desktop mode perhaps.