Use the position of the center of an anchor instead of the top to determine
whether it should be counted as off-screen or not. Since buckets in the bucket
bar are drawn where the center is, this ensures that on-screen buckets merge
into off-screen ones at a natural point, instead of when there is still a gap
between the two (for the "above screen" bucket) or after the on-screen bucket
has gone below the "below screen" bucket.

Fixes #5525