When you have lots of spells learned in your spellbook, sometimes you want to go back to a particular kind of spell to add more of it to your deck.
But especially where Sun- and Shadow spells are concerned, scrolling 6+ pages to add more of the spell you want to your deck can be annoying, and time consuming. I'm sure everyone's had those moments like, WAIT let me finish setting my spell deck (frantic clicking and scrolling ensues).
In my case I kind of haphazardly add 2 of this kind, 3 of that kind and see how it fares.
At the moment, right clicking on a spell within the grid boxes of your spell deck - does nothing.
I suggest that instead of that default behavior, right clicking on a spell currently 'in use' in your spell deck should add more of that same spell to your deck (provided there is room).
Furthermore, right clicking on a spell and holding, should scroll your spellbook to the 'page' that that spell is located on (in your spellbook, that is).
I tried to make my explanation was as clear as possible, lol.