Template part replacement: reduce jumpiness of loading experience

[annezazu]

What problem does this address?

The template part replacement modal shifted to be more of a grid recently #49894. As a result though, the current experience is quite jumpy. Here's what it looks like with TT3:

jumpiness.mov

This gets more jarring the more options you have baked in:

jumpiness.x2.mov

What is your proposed solution?

• Adjust the design to accommodate the loading.
• Add a spinner of some sort.
• Improve the loading itself, related to Refine the Site Editor loading state #35503

[annezazu]

FYI to @jameskoster since I know you've been iterating here.

[jameskoster]

I feel like this is closely related to #35503 and the work there.

A per-pattern loader has been suggested before, but I think that may still result in jumpiness. If so it might be better to display a single spinner until everything has finished loading.