Union of graphs with common or overlapping edges do not create a graph enclosing the entire area

[vivekramcfc]

Consider the following two rectangles:

1. X: 0, Y: 0, Width: 100, Height: 100
2. X: 100, Y: 0, Width: 100, Height: 100

The two rectangles have the edge (100, 0) -> (100, 100) in common. The union of these two produces the following output.

Shouldn't it produce something like this?

Also this: