I have been frustrated with the inability to split inventory in the traveler's chest nor pick it up in my inventory if I can't split it there. I have 13 backpacks of food that are unusable because I can't pick them up!
Alt+Drag the item to an empty inventory space.
Or is the issue that one can't open the backpacks while they're in a container?
I think that it's actually Left Shift by default.
Also, I think the item can be dragged to spaces that aren't empty, but empty or not, what's important is that it's dragged to a space that's in the same window.
As for opening backpacks in containers, you just have to open the Context Menu (Right-Click by default) on a backpack and select "Open".
