Message received while at the bottom of a conversation is not scrolled into view

[levinium]

Checklist

• I can reproduce the bug with the latest version.
• I made sure that there are no existing issues - open or closed - to which I could contribute my information.
• I made sure that there are no existing discussions - open or closed - to which I could contribute my information.
• I have read the FAQs inside the app (Menu -> About -> FAQs) and my problem isn't listed.
• I have taken the time to fill in all the required details.
• This issue contains only one bug.
• I have read and understood the contribution guidelines.

Affected app version

1.9.1

Affected Android/Custom ROM version

Android 17 / GrapheneOS (build 2026091001)

Affected device model

Pixel 10 Pro XL

How did you install the app?

F-Droid / IzzyOnDroid

Steps to reproduce the bug

1. Open a conversation that has enough messages to fill more than one screen.
2. Scroll all the way to the bottom.
3. Receive a message in that conversation.

Expected behavior

The list scrolls down so the newly received message is fully visible.

Actual behavior

The list does not scroll. The new message lands below the fold - often with only its top edge visible - and has to be scrolled to by hand to be read.

It happens both with and without the keyboard open. Conversations short enough to fit entirely on screen are not affected, because stackFromEnd keeps them pinned to the bottom regardless.

Additional information

The relevant code is in ThreadActivity.setupAdapter():

val lastPosition = itemCount - 1
val lastVisiblePosition = layoutManager.findLastVisibleItemPosition()
val shouldScrollToBottom =
    currentList.lastOrNull() != latestThreadItems.lastOrNull() &&
            lastPosition - lastVisiblePosition == 1
updateMessages(latestThreadItems, if (shouldScrollToBottom) lastPosition else -1)

Two things look wrong here:

1. lastPosition - lastVisiblePosition == 1 is the "is the user at the bottom" test, but when the user really is at the bottom that difference is 0, not 1. The condition fails in exactly the case it is meant to catch.

2. When it does fire, the scroll target is lastPosition, which is itemCount - 1 of the list before the update - that is, the message before the one that just arrived.

I have a working fix for this and would be glad to open a PR if this gets the help wanted label.

[levinium]

The fix is ready on a branch if it is useful for triage - a single commit touching only ThreadActivity.setupAdapter(), +11/-5:

main...levinium:Messages:fix/scroll-to-new-message

It treats the user as being at the end of the conversation when the last item is at least partly on screen (or when nothing has been laid out yet, i.e. the first load), and scrolls to the last index of the new list rather than the old one. assembleFossDebug, detekt and lintFossDebug all pass, and I have been running it on a Pixel 10 Pro XL.

I am not opening a PR since the issue is not labeled help wanted - happy to do so if you would like it, or feel free to just take the change.

[chrisdebian]

Noticed this already has a working fix posted below (and the same situation on #877) — both just sitting un-PR'd because neither issue is labeled help wanted yet. Would you be able to add the label to one or both so they can go up as proper PRs?