The direction in which a new window docks to existing ones is defined by the location at which you drop it. To add a new window to an existing tab group, release the mouse button over a docked window's title. To add it to one of the four borders, move the cursor close to this border and release the mouse button there. In any case, yEd will show a grey border while dragging the window which indicates the resulting docking type at the current mouse location.
