Just to note, I discovered you can't append to containers already containing something as soon you exit and enter that map again.

You can drop items in, but they are simply lost!

I only noticed this after losing pretty valuable items (several rifles), well I administered reload but by bad luck closest save was hours ago... And I realized I destroyed entire shipments earlier.

To append to a container on map that wast empty when you last loaded that map you have to take everything out of it first.
So, here goes my tries to have all my inventory nicely sorted....
I hate let items just lying around on ground (and aren't that even invitation to thievery?), so I would like to have extra items nicely organised in chests and lockers, preferably in rooms that were locked (too bad I can't lock them again) but this make that unnecessary hard and tedious to do.

Is this corrected in 1.13 ?
I think I eventually will go for it, but was wanting to play original trough first. And with all the changes, can I instal it and continue the current saved game?

Sounds to me like you do item management while playing in a sector.

I strongly advice you instead use the sector inventory.
In the map view, right mouse on the sector square and select inventory.
Items getting sorted there automatically.

1.13 has ALOT additional hotkeys for item management (for tactical and map/sector inventory mode), check the hotkey list for that.

The only reason to manage item in tactical mode is to lessen the chance does the stupid ai soldiers pick up your stashed weapons.

I know it pointless and tedious, but that exactly what I want to do, I want each and every item in exact that spot on tactical map I want it.

Btw, I discovered this bug not affects all containers, it is true for boxes in Drassen airport near BR delivery box (that is where I discovered it) and chests in church in Drassen mine sector. In contrary, boxes in back room at Tony in San Mona works as expected, I can append to any of them anytime (well, I now quicksave and verify every time anyway).

And yes, I'm gradually giving up my desire to have every item in dedicated box, at first I not realized how much crap you will have (and thinking about crazy option of drop all in 1.13 makes my head spin), still having items lying around in random places causes me great pain (only psychologically of course, there no obvious reason to not let it that way, sadly).

I have found some neat tricks using sector inventory:
- as rather obvious, if you append to a stack of items it remains in place
- if you take item from sector inventory and place back on it, it remains in place, including splitting from stack
- to move item to position your merc is, you have to land it in merc inventory first, then take from there and place in empty cell of sector inventory
- if you take item from merc inventory and swap with item in sector inventory it goes to position on map where that item was; you can now place the item at cursor back in empty cell of sector inventory and it remains in place.

With that in mind it possible to control and sort your sector inventory without moving around (too much). Yes, I know, I have read most of bear's pit forum by now and follow it regularly, and I undoubtedly will give it a try sometime in the future, but since I can't continue my current plain vanilla camping with it (as far I understand saves are not compatible, frankly, as expected by the massive changes done) that won't be anytime soon. On bright side, I can wait for current bugs to be fixed, some secondary moods to catch up with latest additions and so on. I would see 1.13 as slightly different game anyway by this point.

As to inventory management shortcuts and functions... I see why they emergerge and are useful, still I see them as violence against the game, in a way, sadly manually managing the mountains of crap you soon are dealing with is not really an option either.
I still have not actually installed 1.13 so can't comment on anything there, but just reading and thinking I would have attempted slightly different approach to item management probably. Maybe I post some of my ideas in bear's pit once I get some firsthand experience with 1.13 and still feel them relevant.