fix: thread pagination not loading when dragging the scrollbar

[jessicaschelly]

Proposed changes (including videos or screenshots)

Thread pagination was guarded until the user interacted with the message list. However, dragging the custom overlay scrollbar did not count as an interaction because the scrollbar is rendered outside the scrollable viewport.

This change listens for pointerdown on the viewport's parent, where events from the overlay scrollbar are received. The listener is removed during cleanup. This matches the existing room-history pagination behavior in #41098.

The test verifies that:

• Scrolling before a recognized user interaction does not fetch another page.
• Interacting with the overlay scrollbar and reaching the upper pagination threshold fetches the previous page.

Issue(s)

CORE-2512

Steps to test or reproduce

1. Open a channel with a thread containing enough replies to have unloaded pages.
2. Open the thread.
3. Without using the mouse wheel, drag the overlay scrollbar to the top.
4. Confirm that reaching the pagination threshold fetches the previous page.
5. Confirm that wheel, touch, and keyboard scrolling continue to work as expected.

Further comments

Summary by CodeRabbit

• Bug Fixes
  • Improved thread message scrolling behavior so older messages load only after intentional scrollbar interaction followed by scrolling.
  • Prevented unintended message fetches from scrolling alone.
• Tests
  • Added coverage for scrollbar interaction and ensured previous-page requests occur exactly once.

[dionisio-bot]

Looks like this PR is ready to merge! 🎉
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[changeset-bot]

⚠️ No Changeset found

Latest commit: a7c8ef7

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[coderabbitai]

Walkthrough

ThreadMessageList now tracks pointer interaction on the scroll container parent. New tests verify that ordinary scrolling does not fetch older messages, while overlay-scrollbar interaction triggers one previous-page fetch.

Changes

Thread pagination interaction

Layer / File(s)	Summary
Pointer interaction tracking apps/meteor/client/views/room/contextualBar/Threads/components/ThreadMessageList.tsx	The list registers pointerdown on its parent element and removes the previous touchmove listener. Wheel and keyboard listeners remain active.
Previous-page loading validation apps/meteor/client/views/room/contextualBar/Threads/components/ThreadMessageList.spec.tsx	The test setup mocks virtualization, UI components, hooks, context, and message data. The pagination test verifies that scrolling alone does not fetch older messages, while overlay-scrollbar interaction triggers fetchPreviousPage once.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• RocketChat/Rocket.Chat#41098: Applies a similar pointerdown-based interaction gating change to a related virtualized list.
• RocketChat/Rocket.Chat#40998: Covers related ThreadMessageList pagination and scroll interaction behavior.
• RocketChat/Rocket.Chat#40653: Modifies ThreadMessageList.tsx scroll handling for scroll-anchor restoration.

Suggested labels: type: bug

Suggested reviewers: martinschoeler, tassoevan

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the fix for thread pagination when dragging the scrollbar.

Warning

Review ran into problems

🔥 Problems

Errors were encountered while retrieving linked issues.

Errors (1)

• CORE-2512: Request failed with status code 401

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.64%. Comparing base (692be5e) to head (a7c8ef7).

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #41705    +/-   ##
=========================================
  Coverage    68.64%   68.64%            
=========================================
  Files         4164     4164            
  Lines       158957   159264   +307     
  Branches     28160    28282   +122     
=========================================
+ Hits        109113   109334   +221     
- Misses       44678    44771    +93     
+ Partials      5166     5159     -7     

Flag	Coverage Δ
e2e	58.85% <66.66%> (-0.03%)	⬇️
e2e-api	45.73% <ø> (-0.02%)	⬇️
unit	70.53% <100.00%> (-0.03%)	⬇️

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[cubic-dev-ai]

All reported issues were addressed across 2 files

<sub>Reply with feedback, questions, or to request a fix.

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[nazabucciarelli]

we should add a patch changeset and modify the PR title to explain what's being fixed instead of how (https://developer.rocket.chat/docs/pull-requests-tags)