Yes, it seemed the culprit was an accidental narrowing of the content area. I witnessed the fix firsthand after a browser refresh last night, and the area got wider by about 40 or so pixels.
It felt narrower when this banner thing started happening so obviously something pushed the boundary of the image over, and it was probably the narrowing of that column.
During the banner image thing it was showing about 4.5 videos across in channel sections' Horizontal View, now it's back to the full 5 videos across.