Currently, the inventory is one long column and needs to be scrolled, which is a problem, if you want to use an item from the bottom on an item at the top. At the same time, there is a lot of space on the screen that is unused and therefore wasted.
I would suggest the following changes to the inventory:
1 - Move the whole inventory (including "vicinity" and "player") to the left, this creates space for a second inventory column (see file 3), while retaining space at the top to drag unwanted items to. However, if the player has lots of container clothing on them, two columns will still not be enough to display everything at once. Therefore:
2 - Make the columns narrower and add a third inventory column. Currently, the dark grey bars with the container's names (like a backpack or a vest name) are much wider than the actual inventory space they provide, so a lot of space is wasted. I've marked this as red squares in file 4. The only items that currently are full width are weapons with attachments and the miscellaneous clothing items at the very top, both of which can be broken down into several rows (yellow markings in file 4).
3 - Instead of or in addition to #2, you could also turn all the inventory columns into one large area, where each container item is a square with a title and a number of inventory spaces. These containers could then be arranged to best fit the available screen space in the inventory area. The reasoning behind this: Even with the reduced column width, items like the jacket and pants still waste about 30% of available horizontal space (see file 4, purple markings).
4 - Taking #3 one step further, you could also use this concept to display the contents of containers like first aid kits or protective cases without the need to open them. Just make the inventory squares inside those containers a bit smaller, so that you can fit them and and the container name into the quares that the container itself uses. I've sketched this in file 5.
The attached images are also available here: