Fix another case of read markers not updating: Ensure proper double linking of TimelineChunks

[SpiritCroc]

We need both directions so getOffsetIndex() produces correct results in
all cases.

Signed-off-by: Tobias Büttner dev@spiritcroc.de

Type of change

• Feature
• Bugfix
• Technical
• Other :

Content

Ensure that links in both directions are properly set up, so getOffsetIndex() produces correct results.

Motivation and context

I was running in a case where the read marker would not properly update to the most recent of my scrolled-to messages. It turned out that the tracker was thinking of a previously read event to be the most recent scrolled-to one, as LoadTimelineStrategy.getBuiltEventIndex() was returning a lower index than the others. Turned out that the chunk of that event had not linked nextChunk, even though there were more chunks being displayed. This resulted in a wrong result of getOffsetIndex() = 0.

Tests

Scroll around in a room affected by the bug and observe what happens in TimelineViewModel.observeEventDisplayedActions(). Close and re-open the room to see if read marker updated.

Tested devices

• Physical
• Emulator
• OS version(s):

Checklist

• Changes has been tested on an Android device or Android emulator with API 21
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request includes a new file under ./changelog.d. See https://github.com/vector-im/element-android/blob/develop/CONTRIBUTING.md#changelog
• Pull request includes screenshots or videos if containing UI changes
• Pull request includes a sign off
• You've made a self review of your PR
• If you have modified the screen flow, or added new screens to the application, you have updated the test UiAllScreensSanityTest.allScreensTest()

[ganfra]

Makes sense, thanks for this