Here is a few seconds of Debug output during the big initial sync:
D1617548649

Here is a Debug ID during a data sync where the I tried to click on an item to open the attachment, but nothing happens:
D837787141

From a user perspective, it seems weird that the attachment icons (not the files themselves) wouldn't show up in the data sync.

From a user perspective, it seems weird that the attachment icons (not the files themselves) wouldn't show up in the data sync.

Actually, maybe I'm misunderstanding what you're saying. Do you just mean that you don't yet have child items (and that's why it's taking you to the URL)? Because that's true — 5.0 syncs parent items first, and then fills in the child items.

Yeah, so that's largely a function of how the API works, and also from wanting to show ongoing progress during a sync, but we can consider not doing it that way.

The API allows getting all items, getting just top-level items, or getting the children of a given item. Since we want to show ongoing progress, we don't want to waste time fetching child items that we can't yet save, so we start by retrieving batches of top-level items. Once those are all downloaded, we fetch a list of all items, find those we haven't yet downloaded, and start requesting batches of those with the all-items request.

An alternative would be to download a batch of top-level items and then, before moving on to more top-level items, make requests for the child items of each of the top-level items that have them (which we can tell ahead of time). It would result in more requests overall, and so probably reduce the overall speed, but it would mean that child items would appear shortly after a top-level item appeared.

In either case, at-sync-time files wouldn't download until after all data syncing was done, but on-demand download would be available right away for the child attachments that had been saved.

The performance element probably isn't that big a deal — there are bigger ways we can optimize the sync anyway — so it's a question of what's more useful (and less alarming) to have sooner: complete items with attachments (useful if you want to open attachments) or all top-level items (useful if you want to search for something, cite something, or load a URL). You're probably right that the former would be more logical from a user perspective.

I think if at least the record of the child items could be retrieved so that the icon could be displayed (and the item double clicked to issue an interrupt file download request if needed), that would be preferred. With a large library, I worried that something had gone wrong with the syncing and that the attachments wouldn’t show up.