In the Chromosome Browser — One-to-many, sometimes you will see a region of DNA where all colored bars appear, but there is no triangulated segment indicated there. This means you share DNA segments at the same genomic position with multiple DNA Matches, but they do not share that segment with each other.
Triangulated segments are segments that all of the selected DNA Matches share with each other. Every segment between two people is triangulated, but segments that seem to be shared by three or more people are not necessarily triangulated.
If you and one DNA Match share a segment on your maternal chromosome, and you and a different DNA Match share a segment at the same genomic location on the paternal chromosome, those two DNA Matches will likely not share a segment at that genomic location with each other, and they might even be unrelated to each other. In this case, that segment is not triangulated and will not be framed.
This is because each chromosome pair is represented by a simplified single stripe in the Chromosome Browser; there is no graphical differentiation between the chromosome within each pair that was inherited from the mother and the one that was inherited from the father.