I guess the idea behind this is that you can see in which inventory window your Shift-transferred items will go to. (... In some cases)
As usual, it kinda works. But the dev's forgot that the player inventory is kinda special. (other inventories are now called containers)
The special nature of the player inventory is that it will 'always' be the default target when shift-moving stuff from other containers.
And that ones you click an item in your inventory. The highlight on the previous container moves to the player inventory, which might seem logical but its not in relation to the player inventory being the default target for all containers.
So: Assuming you had 2 containers open, they now are both not highlighted. Ergo: Now the client in not showing which container will be the target container when moving stuff from your inventory.
Or, of course, dev's added this highlight feature for an other reason. One that's beyond me though, until than I'm going to play some memory games (now you see me, now you don't.