Double clicking on a item doesn't auto-stack the items in creative mode, but it still works in survival mode.
Related issues
is duplicated by
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Confirmed for
1.8.2-pre6
@ Marcono1234
Can't confirm. Works fine for me.

Well tested it again and cannot confirm neither. I experience instead a strange bug, where it won't stack the items in survival and creative, but I can't figure out the reason, sorry.
In 1.8.1 it still doesn't work