Comment 11 for bug 1073724

Reproduced, but the conditions are narrow. The barrier must align with the edge of a monitor and the movement _vector_ must not overlap with the barrier.
e.g. if you're in that corner, moving by 1/1 will block, moving by 1/2 won't block.

Cause is that the fixes code that calculates whether a barrier is blocking doesn't know about screen edges. So the picture the fixes code sees for a movement a to b is

                a X
                 \X
                  \
                   b
Which is permitted since the movement goes just past the barrier. The RandR
code to clamp to monitors sees this picture:

 +----------------+---------------------+
 | | |
 | | |
 | | |
 | a | |
 +---------------\+ |
                  \ |
                  |b |
                  +---------------------+

Which too is allowed since the destination exists in RandR space. The two
operations are quite separated in the server, merging them is tricky.

As a workaround for now, I suggest either extending the barrier to the bottom of the second monitor like this:

 +----------------+---------------------+
 | | |
 | | |
 | | |
 | X |
 +----------------X |
                  X |
                  X |
                  X---------------------+

or, alternatively, to create a horizontal barrier in that corner that shares
the same endpoint with the vertical one.