[blue][red], you want to make sure you round their shared edge either to the left or to the right. Otherwise, there will be a gap between them. You need to make sure you round their shared edge the same way consistently too, or it will jump back and forth as you resize a window and produce a noticeable jitter.

In this case I would probably do something like:

    red.left = blue.right;

(Or vice-versa, depending on which edge you are positioning against).