When, for example, you have a terminal behind a maximized browser, and you move
another terminal around, the latter snaps to the first terminal, even though it
is hidden. Increasing the window snap margin makes this more apparent.
Steps to Reproduce:
1. Open one small window.
2. Open a maximized window.
3. Open another small window and move it around.
The third window snaps to completely invisible windows.
Windows should only snap to at least partly visible window borders.
This is not really a bug, it's more of a design decision. Computing what edges
are truely visible for every window, at every single move of a window would
waste a lot of resource.
None of the WM I know do that either.