1: if you put a few items of the same sort and damage value
in your creative inventory and you double click on one of them
it won't stack into one stack!

if you do in survival though it will work!

2: if you open minecraft (click play)
and then at the menu press the [-] (minimize button) at the right side
it will shutdown minecraft and re-open the launcher if your option is
set to "close launcher when minecraft starts".