Only consider window edges that transitively touch each other

[Flupp]

There is the following longstanding deficiency:

Currently, not only snapped window edges are considered as connected, but even edges which are only on the same row/column are considered as connected. I have not decided yet if this is a bug or a feature.

A not too inefficient algorithm to find exactly those windows that transitively touch the moved edge might look as follows; we assume a vertical edge is moved:

• Let W be the set of all windows.
• Let S = {the resized window}.
• Let TODO = W ∖ S.
• Let x be the current horizontal position of the moved vertical edge.
• Let l be the vertical position of the lower edge of the resized window.
• Let u be the vertical position of the upper edge of the resized window.
• While TODO ≠ ∅:
  • Choose c ∈ TODO.
  • TODO = TODO ∖ c.
  • If c has a vertical edge that has horizontal position x and that touches the interval (l, u), then:
    • Let S = S ∪ {c}.
    • Let l = min(l, vertical position of the lower edge of c).
    • Let u = max(u, vertical position of the upper edge of c).
    • Let TODO = W ∖ S.
• Now S should contain exactly those windows that transitively stick to the moved edge.

Runtime presumably is in O(|W|²).