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Ability to select multiple window borders and resize them at the same time. For example, if you had two windows side by side and you wanted one to take up more of the screen, just select the two borders side by side and they'll both resize together. Also, you could do it with more than two borders.

Suppose you have a screen setup where one window is taking up half of the horizontal space and all of the vertical space. In the remaining half, you have two windows, each at 50% vertical. By selecting the borders in between the two smaller ones and the bigger one, you can the screen percentage of all three at once.

This is how the tiling window management in 4.5 is supposed to work.