Corey Berla authored Jan 15, 2024 and António Fernandes committed Jan 16, 2024

After the last commit, get_first_visible() file is only used for
thumbnail prioritization logic. prioritize_thumbnailing_on_idle()
is very inefficient and over complex as it constantly iterates
through the bound items to see what is visible.

Instead, let's rely on an GTK implementation detail: list widget
children outside the scrolled viewport area are not mapped. With
this assumption, we can connect to ::map in the cells needing a
thumbnail.

The only catch is that it maps more items initially than are
visible, so also connect to ::unmap revert the prioritization for
items that are mapped and immediately unmapped.