Fill Empty Gaps with Dummy Element

This was discussed before here: https://github.com/metafizzy/packery/issues/131
While apparently it's more or less possible to fill empty gaps with dummy elements, it's ... rather hard, for a beginner like me. Would it be possible to add an easier way of doing this? Or a shortcut of some kind?

Start by reading the linked discussion in issue #131 and tracing how Packery currently handles dummy elements. The issue names no files or tests, so identify the relevant public entry point before assessing the design. Done would mean a clearly defined, easier supported way to fill empty gaps with dummy elements.

Written by the indexing model from the issue text.