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. Reproducible: Always Steps to Reproduce: 1. Open one small window. 2. Open a maximized window. 3. Open another small window and move it around. Actual Results: The third window snaps to completely invisible windows. Expected Results: 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.