This sure looks useful...if I could figure out how it works. By right-clicking on a backpack as instructed, it opens my player inventory. If I shift-right-click on a stack in the inventory, it transfers it to the backback. I can see the tiny icons on the bag and hovering over the bag shows me the contents. But right-clicking on the backpack simply takes me back to my player inventory and does not open up the backpack to retrieve the items inside it.

What am I doing wrong?

Further comment:
There appears to be a conflict between this Backpack mod and the Big Bag mod in the fact that they both apparently share a designated screen box for player inventory, with the Big Bag mod taking priority. As the Big Bag capacity is researched up, it leaves less and less space for the BackPack contents display, to the point that when Big Bag reaches level 5, there is no space left for the BackPack contents display to open.

I have not found a way to extend the player inventory screen box to see the BackPack contents at this point.

Both mods are useful, and it would be good to have the BackPack contents in a separate box of its own.