test(timeline): add regression tests for cursor-anchored zoom (#847)

[TimeToBuildBob]

Summary

PR #860 fixed timeline zoom drifting away from the cursor position (issue #847) but included no automated tests. This PR adds 8 unit tests to guard against that regression being reintroduced.

What the tests cover

• preferZoom: true is present in the vis-timeline options (prevents the pan-after-zoom drift)
• onHorizontalWheel returns early when this.timeline is not yet initialised
• onHorizontalWheel returns early when vertical scroll dominates (deltaY >= deltaX)
• onHorizontalWheel pans the window by the correct amount when horizontal scroll dominates
• preventDefault and stopImmediatePropagation are called on every handled event
• DOM_DELTA_LINE mode scales deltaX by 40 px/line
• DOM_DELTA_PAGE mode scales deltaX by 800 px/page

Approach

The tests call component methods directly on a mock vm object (VisTimeline.methods.onHorizontalWheel.call(vm, event)) and stub vis-timeline/esnext so they run in jsdom without a real browser layout or resize-observer stack. All 8 tests pass (npm test -- --testPathPattern VisTimeline).

Test plan

• npm run test:unit -- --testPathPattern VisTimeline — all 8 tests pass
• Full suite (npm test) remains green

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 35.59%. Comparing base (8768475) to head (75c4f72).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #866      +/-   ##
==========================================
+ Coverage   35.50%   35.59%   +0.09%     
==========================================
  Files          36       36              
  Lines        2152     2152              
  Branches      417      398      -19     
==========================================
+ Hits          764      766       +2     
- Misses       1309     1365      +56     
+ Partials       79       21      -58     

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[greptile-apps]

Greptile Summary

Adds 8 regression tests for the cursor-anchored zoom fix (PR #860 / issue #847), covering the preferZoom option and the onHorizontalWheel handler's early-exit and panning logic. The tests call component methods directly on a mock vm and stub vis-timeline/esnext so they run cleanly in jsdom.

• preferZoom: true assertion verifies the option is present in data().options, directly guarding against that field being accidentally dropped.
• onHorizontalWheel tests cover all four branches (null timeline, vertical-dominated scroll, equal-magnitude scroll, and horizontal-dominated scroll) plus event propagation suppression and both non-pixel delta-mode scale factors — the computed diff values in the tests match the implementation's formula exactly.

Confidence Score: 5/5

Test-only change with no production code modified; the new tests correctly model the existing implementation and run in the established jsdom project.

All changes are in a new test file. The test math was verified against the actual onHorizontalWheel implementation — window offsets, delta-mode scale factors, and early-exit conditions all match. The vis-timeline mock is minimal and correct. No production paths are affected.

No files require special attention.

Important Files Changed

Filename	Overview
test/unit/VisTimeline.test.js	Adds 8 unit tests covering preferZoom option and onHorizontalWheel guard/pan logic; test math correctly matches the implementation, vis-timeline is cleanly mocked for jsdom, two minor assertion-completeness gaps.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[wheel event fired] --> B{this.timeline null?}
    B -- yes --> Z[return early]
    B -- no --> C{abs deltaY >= abs deltaX?}
    C -- yes --> Z
    C -- no --> D{deltaMode?}
    D -- DOM_DELTA_LINE --> E[deltaX x= 40]
    D -- DOM_DELTA_PAGE --> F[deltaX x= 800]
    D -- DOM_DELTA_PIXEL --> G[no scaling]
    E --> H[diff = deltaX/120 * windowMs/20]
    F --> H
    G --> H
    H --> I[setWindow start+diff end+diff]
    I --> J[preventDefault + stopImmediatePropagation]

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_{Reviews (2): Last reviewed commit: "test(timeline): address Greptile review ..." | Re-trigger Greptile}

[TimeToBuildBob]

@greptileai review

[TimeToBuildBob]

CI is green and the PR is mergeable.

I tried to squash-merge it from the monitoring lane, but the TimeToBuildBob token doesn't have MergePullRequest permission on ActivityWatch/aw-webui, so this is waiting on a maintainer merge.

[TimeToBuildBob]

Bumping — this has been idle 5 days. CI is green, ready for review/merge.