Existing partial messages remain incomplete after changing Fetch messages up to to Any size

Checklist

• I have used the search function to see if someone else has already submitted the same bug report.
• I will describe the problem with as much detail as possible.

App

Thunderbird for Android

App version

20.1

Where did you get the app from?

None

Android version

16

Device model

Samsung Galaxy S25 Ultra

Steps to reproduce

1. Open a message in a custom IMAP folder that displays the Download complete message action.
2. In the account settings, change Fetch messages up to to Any size (no limit).
3. Enable automatic display of remote images.
4. Close and reopen Thunderbird Android.
5. Open the same partial message.

Expected behavior

Existing partial messages should either be downloaded automatically after changing the fetch-size limit, or the app should offer an action to download all existing partial messages for the account or selected folder. At minimum, the setting should clearly explain that it affects only future downloads.

Actual behavior

The already-partial message still displays Download complete message. Its body and images are unavailable until that action is pressed for each message. Changing the account limit to Any size and reopening the app does not complete messages already stored as partial.

This is particularly problematic for messages that server-side rules move directly into custom non-Inbox folders.

Logs

No log attached for privacy. No message content or account details are included.

The payload names no files or tests; start by tracing the account setting for “Fetch messages up to” and the partial-message “Download complete message” action. Reproduce the change to “Any size” with an existing partial message in a custom IMAP folder, then verify that the resulting behavior matches the selected resolution for existing messages.