Refactor packages/app/src/pages/session/timeline/rows.ts

[micahfna]

P1B: Starter Task: Refactoring PR

1. Issue

Link to the associated GitHub issue:
#193

Full path to the refactored file:
packages/app/src/pages/session/timeline/rows.ts

What do you think this file does?
It builds the list of "rows" rendered in a session's message timeline UI — grouping raw user/assistant messages into turns and converting each turn into a sequence of typed row objects (user message, assistant response parts, thinking/retry/error indicators, diff summaries, etc.) that the timeline component then renders.

What is the scope of your refactoring within that file?
Timeline.constructMessageRows

Which Qlty‑reported issue did you address?
Function with many parameters (count = 9) in constructMessageRows, reported at packages/app/src/pages/session/timeline/rows.ts:101.

2. Refactoring

How did the specific issue you chose impact the codebase’s maintainability?
With 9 positional parameters, callers had to get argument order exactly right with no compiler help distinguishing similarly-typed booleans (showReasoning, isActive, inlineComments), making the call site error-prone to read and easy to break silently when adding or reordering parameters.

What changes did you make to resolve the issue?

Introduced a ConstructMessageRowsParams object type bundling all 9 fields, changed constructMessageRows to accept a single params object destructured at the top of the function body, and updated the internal call site in constructSessionMessageRows to pass a named object literal instead of positional arguments.

How do your changes improve maintainability? Did you consider alternatives?

The call site is now self-documenting (isActive: turn.user.id === activeMessageID instead of an unlabeled positional boolean) and safe to reorder or extend without risk of argument-position mistakes; I considered splitting the function into smaller helpers instead

3. Validation

How did you validate that the change is correct?
I added packages/app/src/pages/session/timeline/rows.test.ts, five unit tests that call Timeline.constructMessageRows directly with the new options object, each isolating one field (index, inlineComments, isActive/status/showReasoning, status: "retry", summary.diffs) and asserting the exact resulting rows; I also re-ran the pre-existing rows-current.test.ts (5 tests) which exercises the refactored call site end-to-end through the public constructSessionMessageRows entry point. All 10 tests pass, and bun test --coverage confirms the changed lines (the call site and the new destructuring block) are executed.

Attach a screenshot of the test coverage showing the lines were executed by the tests.

Attach a screenshot showing the tests that cover the change passing during CI

Attach a screenshot of qlty smells --no-snippets <full/path/to/file.ts> showing fewer reported issues after the changes.