[Feature]: Add a setting for default changed-files expansion in chat UI #2158

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/web

Problem or use case

The chat timeline Changed files section is not configurable and can become very noisy when a turn includes a large diff.

A common case is updating or rebasing a branch, which can make the assistant message show a massive changed-files list in the chat UI. When that happens, the expanded file tree can dominate the thread and push the actual conversation content out of view.

Examples:

• I update my branch from main and a turn now shows a very large changed-files list.
• A rebase or merge touches many files and the chat becomes hard to scan because the changed-files block is expanded.
• In some workflows, I want to read the assistant response first and only expand changed files when I explicitly need to review them.
• In other workflows, I may want changed files visible immediately.

Proposed solution

Add an app setting that controls the default expansion behavior of the chat timeline Changed files section.

Suggested options:

• Expanded
• Collapsed

This setting should only affect the initial state when a given changed-files block is first shown.

If there is already a persisted user override for a specific thread/turn, that existing state should continue to win over the global default.

Why this matters

Different users want different review defaults.

A configurable default would:

• reduce visual noise for users who often work with large diffs
• avoid forcing a single opinionated default on everyone
• make the chat UI more usable after rebases, merges, or large branch updates
• still preserve the existing one-click expand/collapse interaction

Smallest useful scope

Add a single app setting in apps/web for the initial default state of chat Changed files blocks:

• Expanded
• Collapsed

The setting only affects first render when no per-thread/turn override already exists.

No redesign is needed.
Existing manual expand/collapse behavior can remain unchanged.

Alternatives considered

• Make changed files collapsed by default for everyone.
  • Simpler, but opinionated and likely to create the opposite request from users who prefer expanded-by-default behavior.
• Only auto-collapse when the changed-files list exceeds some threshold.
  • Could be useful, but adds more heuristics and edge cases than a simple user preference.
• Keep the current behavior and rely only on manual collapsing.
  • This does not solve the repeated “massive expanded list” problem on initial render.

Risks or tradeoffs

• Adds one more app setting to maintain.
• Need to define how the global default interacts with existing persisted per-thread/turn expansion state.
• The setting should be clearly scoped to the chat timeline Changed files section so it is not confused with diff panel behavior.

Examples or references

• [Feature]: Add collapsible diff #1122 / PR feat(diff-panel): add per-file collapse toggle in diff panel #1125 adds per-file collapse in the diff panel
• PR Persist changed-files expansion state per thread #1858 persists changed-files expansion state per thread, but keeps the default behavior unchanged

This request is specifically about adding a user setting for the assistant message Changed files section in the chat timeline.

Contribution

• I would be open to helping implement this.