Improving background sync (download)

I found similar issues that were closed but are still unsolved AFAIK:

• https://github.com/nextcloud/ios/issues/1545
• https://github.com/nextcloud/ios/issues/1099

Steps to reproduce

This is just an example. Different usage scenarios are possible.

1. Mark a large file (e.g. >2GB) as available offline.
2. Wait for transfer to be e.g. at 50%.
3. Switch to another app.
4. Switch back to nextcloud.
5. Download restart from 0%.
6. Profit!

Expected behaviour

Two possible outcomes:

• Download should resume from 50%.
• Download should be run in the background.

Actual behaviour

• Download is restarted from scratch.

Screenshots

If applicable, add a screenshot showing the issue.

Logs

If applicable, you can post the iOS app or server logs (removing any sensitive information).

Reasoning or why should it be changed/implemented?

Environment data

iOS version: e.g. iOS 14.4.1
16.5.1

Nextcloud iOS app version: see More > Settings
4.8.3.1

Server operating system:

Web server: Apache, nginx

Database:

PHP version:

Nextcloud version: see Nextcloud admin page
27.0.0

Start by reproducing the download interruption on iOS 16.5.1 with Nextcloud iOS 4.8.3.1, then trace the app's offline-download and background-transfer entry points. Done means a large offline download either resumes from its previous progress or continues in the background without restarting from zero.