Hi, I am new and do not know what that is or how I activate it :-)
Hi. This is a mod. You can download it directly here on the forum: viewtopic.php?f=11&t=12767&hilit=Unified+inventory or install it via the in-game content db installer. The second way is easier for newcomers, you just start up the game, switch to "Content" tab, then the install button.
When you download that, you press "T" key in the game and your new inventory is activated.
I hope the next 1.2.0 version will be released soon (that bug that you showed is already fixed long ago in my dev-branch on the github), just now I suspended work on this mod as I was busy with other things. I intend to continue it.
The mod has been declared as abandoned now. No next features/bugfix releases. All my attention and efforts are switched to new mod "MultiDecor" as a overhaul and sequel of this one: viewtopic.php?f=9&t=28583