mcduck - your .capx is incorrectly set up, it has lots of separate tiled background objects. This will definitely show seams between tiles. You should just have one tiled background object and stretch it to cover the whole layout. That will not show seams on most browsers.
There are bugs with tiled backgrounds on both CocoonJS and directCanvas; I've reported bugs to both of them, but we have to wait for them to fix it. In the mean time be sure to contact Ludei or appMobi about bugs which only occur on CocoonJS or directCanvas, since we cannot do anything about the problem other than just forward your report directly on to them.
Also:
lso, I placed a ring of tiled background objects around the level. So they are not visible and do not lie inside the level, they simply surround the level. In chrome, the object outside the level bleeds into the frame by 1 pixel.
Can you provide a .capx which reproduces this issue as well?