If you look at ~0:19 - 0:20 in the DR video, you will notice that Autopause is a setting... inclusive of being isolated to settings or being in place for menus, etc. Though, to be clear, if you have autopause on in inventory, etc. it will make building... and really anything else.... while inventory is open impossible.
i always use autopause in the current version.
why would i fight or build with the inventory open? do people actually do that? wouldn't the UI block your view?
i figured that's what the hotbar was for... i just put everything i am currently using in the hotbar and just switch to what i need.
also, if i need to use potions, i just press a single button which i have set to use all my buffs at once, or to heal.