When in Edit Layout mode, the panel which runs across the bottom (housing inventory items) is currently from left to right horizontally.
Making it vertical would provide a variety of visual and functional benefits:
• Most monitors have a wider aspect ratio; a vertical menu provides a more pleasing and efficient use of the space available, rather than blocking a larger part of the screen and limiting the ratio, similar to a movie in "letterbox" edition.
• The number of inventory items will very rapidly expand, which will become impractical to scroll across, even with expanding menu/file system. Using a file tree will require an even taller portion across, limiting the view further.
• Most developers have a second monitor, please consider a way to open/detach the Editor and other UI elements as a separate window, a type of developer mode, so that it could be placed on the other monitor with all dev tools.
• The thumbnail of the items are huge in the inventory, so I am thinking they may eventually become a true thumbnail, though a mouse over with a pop up of the item or image would be extremely useful. Using that type of method would also allow the thumbnail in the general inventory list to be removed, using small item type icons instead (as in the SL inventory), allowing the inventory window to become smaller.