When using a secondary monitor with my laptop, I position the secondary monitor  to be above the built-in primary monitor and share a common right edge. The primary monitor still has all four sides as snappable, but the upper monitor behaves oddly:

- Windows on the upper screen cannot have their left edge moved beyond an invisible boundary to the left of my bottom screen's left edge. There is still plenty of room.
- Windows can be resized to fill more of the upper monitor by dragging the bottom-right corner.
- A window resized by the upper-left corner cannot move its left edge beyond the "boundary": the window shrinks from the right edge in, which is unexpected.

From this, I assume that the left edge of my primary monitor is being extended upwards onto the upper monitor. The WM treats this boundary as a screen edge for the upper monitor, even though it is only a boundary for the lower monitor. The lower monitor behaves as expected.