Repro And Fix

repro-and-fix

A user pointed you at a GitHub issue and asked you to reproduce + fix it. Run the workflow below. The whole point is that the dev loop is self-contained: you build the repro, launch the sample at that repro automatically, verify the bug yourself (screenshot or logs), fix it, verify the fix, and commit. Don't ask the user for visual confirmations you can get yourself.

Shared with Copilot Coding Agent. .github/copilot-instructions.md links here as the canonical issue → reproduce → fix → PR workflow. Edit this file when changing the workflow; keep the Copilot doc as a thin pointer to avoid drift.

When this skill applies

• "Let's work on https://github.com/Live-Charts/LiveCharts2/issues/N"
• "Reproduce issue #N and propose a fix"
• "Investigate this bug report: "
• A user paste of an issue URL with no other instruction

If the user is asking about theory ("why does the gauge use a doughnut geometry?") or a non-issue task (refactor, doc update, perf), this skill is not the right tool — fall back to general workflow.

Pick the platform that matches the issue

Read the issue's labels and repro snippet to choose. The workflow is the same on every XAML platform — only paths and launch commands change.

If the issue is platform-agnostic (a core engine bug), prefer Avalonia for the fastest dev loop on Windows (no WindowsAppSDK / packaged-build overhead).

Project conventions you must follow

These are durable rules from ~/.claude/projects/.../memory/MEMORY.md. Re-read the relevant memory file if uncertain — these are not optional:

• Worktree per issue. Create ../LiveCharts2-fix-<issue>-<slug> first (feedback_worktree_per_issue). Don't work in the primary tree.
• One concept per commit. Fix and regression test go in separate commits. The regression-test commit must fail when the fix is reverted — verify this before pushing (feedback_commit_separation).
• Snapshot tests for render-path changes. If your fix touches paint, layout, z-index, or draw order, run tests/SnapshotTests/ before claiming done — unit tests have missed catastrophic regressions (feedback_run_snapshots_for_render_changes).
• Update docs when public API changes in the same change set as a separate commit (feedback_update_docs_for_api).
• Check Directory.Build.props for <LiveChartsVersion> and use that value in user-facing text (project_current_version).

Workflow

1. Read the issue and decide if it still reproduces

gh issue view <N> --repo Live-Charts/LiveCharts2 --json title,body,labels,state,comments

Pay attention to comments, not just the body. Issues commonly evolve:

• The original symptom may already be fixed in main (commenter says "newer releases fixed this") and the current bug is a downstream observation. This is exactly what happened with #2008 — the GaugeValue TemplateBinding bug in the title was already resolved; the live bug was a CornerRadius=0 override in the comments.
• Multiple users may report different things in the same thread. Identify the most recent reporter's actual symptom.

If you suspect the title bug is already fixed: build the original repro verbatim and verify first, before doing diagnosis. Cite the commenter who said it's fixed in your PR description.

2. Set up the worktree

git worktree add ../LiveCharts2-fix-<issue>-<short-slug> -b fix/issue-<N>-<short-slug>

All edits, builds, and runs happen in the worktree. The primary tree stays clean for other work.

3. Build the repro view (XAML platforms)

The repro convention is a sample view inside the platform sample app:

samples/<Platform>Sample/VisualTest/Issue<N>Repro/View.<axaml|xaml>
samples/<Platform>Sample/VisualTest/Issue<N>Repro/View.<axaml|xaml>.cs

Match the issue's repro setup as closely as you can — same ControlTemplate shape, same property values, same paints. Don't simplify until you've confirmed the bug fires; over-minimization can accidentally avoid the trigger.

Existing repros to mirror:

• samples/AvaloniaSample/VisualTest/Issue1986Repro/ — TabControl + ScrollViewer
• samples/AvaloniaSample/VisualTest/Issue1417Repro/ — GeoMap reattach

The View's code-behind should expose helpers that a Factos test can call (e.g. FindTemplatedGaugeSeries()). This is how the regression test hooks in without going through the visual tree.

Do not commit changes to samples/ViewModelsSamples/Index.cs for repro views. Factos navigates to a sample by path via LVC_SAMPLE regardless of whether the path is listed in the index, and the index is shared across every platform sample. If you add VisualTest/Issue<N>Repro there and the view only exists in samples/AvaloniaSample/, every other platform sample (WPF, MAUI, WinUI, Uno) will throw when it loads the index. Leave the file untouched on your branch.

If it's genuinely easier for your local dev loop to pick the repro from the sample selector dropdown rather than via LVC_SAMPLE, edit Index.cs temporarily — but revert it before committing. git restore samples/ViewModelsSamples/Index.cs before each commit, or just don't stage the file.

4. Auto-launch the sample at your repro

All XAML platform samples read LVC_SAMPLE at startup and load that sample instead of the default landing screen. Set it before launching:

# bash / WSL
LVC_SAMPLE=VisualTest/Issue<N>Repro \
  ./samples/AvaloniaSample/Platforms/AvaloniaSample.Desktop/bin/Debug/net10.0/AvaloniaSample.Desktop.exe

# PowerShell
$env:LVC_SAMPLE = "VisualTest/Issue<N>Repro"
.\samples\WPFSample\bin\Debug\net10.0-windows\WPFSample.exe

Build first if needed: dotnet build samples/<Platform>Sample/... -c Debug.

5. Verify the bug yourself

Two paths — pick based on the bug type:

A. In-app screenshot for visual bugs (color, shape, position, rendering artifacts). Avalonia/WPF/WinUI samples support LVC_SCREENSHOT: when set, the sample renders its main window via the framework's native RenderTargetBitmap after a short delay (default 3 s, override with LVC_SCREENSHOT_DELAY_MS) and exits. No external tooling, no race against a screenshot tool launching too early. The capture is scaled to the window's physical pixels (RenderScaling on Avalonia, VisualTreeHelper.GetDpi on WPF) so HiDPI displays produce a sharp PNG that matches what's on screen:

LVC_SAMPLE=VisualTest/Issue<N>Repro \
LVC_SCREENSHOT=$(pwd)/.claude/screenshots/<N>-before.png \
  ./samples/AvaloniaSample/Platforms/AvaloniaSample.Desktop/bin/Debug/net10.0/AvaloniaSample.Desktop.exe

# PowerShell
$env:LVC_SAMPLE = "VisualTest/Issue<N>Repro"
$env:LVC_SCREENSHOT = "$pwd\.claude\screenshots\<N>-before.png"
.\samples\WPFSample\bin\Debug\net10.0-windows\WPFSample.exe

After it exits, Read the PNG. You're multimodal — describe what you see, compare to a control sample (often Pies/Gauge1 or General/FirstChart for "what should this look like"). If the bug is subtle (a few pixels), make the repro larger (Width=380, Height=380) so it's clearly visible.

Fallback for MAUI/Uno or other-OS edge cases: use the OS-appropriate helper script. They all take <ProcessName> <OutPath> [WaitSeconds] and poll for the target window before capturing:

The Linux script tries X11 → sway → GNOME in priority order; install the matching package if none are present (the script prints the apt/dnf hint). Use these only when LVC_SCREENSHOT isn't supported on the target platform (MAUI, Uno) — for Avalonia/WPF/WinUI prefer in-app capture.

B. Console logs for state/dataflow bugs (binding doesn't fire, value doesn't propagate, event order is wrong):

Add temporary Console.WriteLine calls along the suspected path:

Console.WriteLine($"[issue<N>] OnPropertyChanged: {change.Property.Name} old={change.OldValue} new={change.NewValue}");

The platform sample writes to stdout when launched from the terminal. Run via the bash tool with run_in_background=true (no LVC_SCREENSHOT so the app stays open), wait a few seconds, kill the app, then grep the captured output file. Remove the diagnostics before committing.

If neither approach decisively confirms the bug, escalate to the user with one well-framed question, not a chain of polling questions. Example:

"The before-fix capture shows the gauge endcaps as flat, contrary to the issue's screenshot. Did you toggle CornerRadius after the screenshot was taken in the original report?"

6. Diagnose the root cause

Trace from symptom backward through the rendering/data pipeline. Cross-check against MEMORY.md — many bugs in this repo have related ancestors (z-index issues, theme overrides, motion-property defaults, source-gen behavior). If you find a matching memory note, use its diagnosis as a starting point but verify against current code; memories go stale.

Common gotchas in this codebase:

• Theme overrides user-set values when the user's value equals the DP default — Avalonia skips OnPropertyChanged and _userSets never gets populated. XamlGaugeSeries.EndInit syncs IsSet DPs as a remediation; if you see this pattern in another wrapper, the same fix shape applies.
• Series controls aren't in the visual tree. OnInitialized and TemplatedParent may not fire/be set the way you'd expect. Use EndInit instead.
• Theme rules run from the chart engine measure pass, not at construction. _isInternalSet + _userSets is the existing precedence mechanism — don't reinvent it.
• Motion properties animate from a stale "from" value. A value flip from N→0 may render N for one frame.

7. Write the regression test first

Test-first: by the end of step 6 you understand the root cause and the observable that would catch a regression. Write the test now, before applying the fix, and run it — it must fail for the right reason. A test written after the fix risks passing against both versions of the code; a test that fails first proves it's gated on the broken behavior, and you skip the awkward "comment out the fix, rebuild, re-run" verification dance.

Test choice depends on the bug nature:

• Programmatic state assertion (the bug is in state propagation, not pixels): Factos test in tests/SharedUITests/. Navigate to the repro, query a helper method on the View that exposes the state under test, assert. Gate by #if <PLATFORM>_UI_TESTING if platform-specific (most XAML-platform-quirk bugs are).
• Pixel-perfect rendering: snapshot test in tests/SnapshotTests/.
• Animation trajectory (the bug is in per-frame interpolation, not the final value — a bar pops to target instead of growing, a slice doesn't sweep, a transition runs at the wrong speed, a refactor risks changing the motion shape of an existing series): JSON animation baseline under tests/CoreTests/AnimationBaselines/ driven by tests/CoreTests/Helpers/SeriesAnimationCapture.cs. The helper steps CoreMotionCanvas.DebugElapsedMilliseconds, captures the visual's geometry at each sample timestamp, and AssertTrajectoryMatches compares the trajectory to a committed .json. First run writes the new trajectory to AnimationBaselinesNew/ and fails with a "review and commit" message — inspect the file, then move it into AnimationBaselines/. Re-baseline the same way: delete the JSON, re-run, move from New/ back. On mismatch the helper also drops [EXPECTED]/[RESULT] files into AnimationBaselinesDiff/ so you can diff them directly. Caller must use EasingFunctions.Lineal + fixed MinLimit/MaxLimit so interpolation is deterministic; the existing tests/CoreTests/SeriesTests/*AnimationTests.cs files are the templates. Prefer this over a plain MSTest whenever the observable is "how the value changes between two states" — a single before/after assertion can't catch a regression that only mis-paints the intermediate frames.
• Pure C# logic (chart engine, motion, math): MSTest in tests/CoreTests/.

Run the test and confirm the failure message matches the symptom you saw in step 5 — not, for example, a setup error or NullRef from missing test infrastructure. If it fails for an unrelated reason, the test isn't yet catching the bug; fix the test before moving on.

To run a single Factos UI test:

# In tests/UITests/Program.cs, set: var appToRun = "<platform>-desktop"
# (avalonia-desktop / wpf-net10 / winui / maui / uno).
dotnet build tests/UITests/UITests.csproj -c Debug
cd tests/UITests/bin/Debug/net10.0 && dotnet UITests.dll
# Revert the appToRun change before committing — it's a local dev-loop tweak.

8. Apply the fix

Smallest possible change. Don't refactor surrounding code. Don't add comments unless the WHY is non-obvious — but if the bug is a subtle interaction (Avalonia behavior, theme precedence, source-gen quirk), a comment explaining why this fix is necessary is warranted because future readers won't reconstruct it from memory.

9. Verify the fix

Re-run the failing test from step 7 — it must now pass. Then re-run step 5 with the fix in place to capture an "after" screenshot at a sibling path and compare visually:

LVC_SAMPLE=VisualTest/Issue<N>Repro \
LVC_SCREENSHOT=$(pwd)/.claude/screenshots/<N>-after.png \
  ./samples/<Platform>Sample/.../<Platform>Sample.exe

Read both images. Don't claim the fix works without comparing. The test passing is necessary but not sufficient for visual bugs — pixel-equivalent output can still look subtly wrong if the assertion was too narrow.

10. Commit and PR

You developed test-first, but commit fix-first. Two commits, in this order:

1. fix(<scope>): <one-line summary> (#<N>) — only the fix.
2. test(<scope>): regression for <symptom> (#<N>) — only the test + repro view. Do not stage samples/ViewModelsSamples/Index.cs; see step 3 for why.

Why fix-first commit order despite test-first development: every commit on the branch should be individually green so git bisect lands on a usable state. If the test commit landed first it would be a failing-test commit that bisect would (correctly) flag as broken. Fix-then-test keeps both commits green while still satisfying the project rule that reverting the fix must surface the regression — revert commit 1 alone and the test from commit 2 fails.

Commit message style follows recent history (git log --oneline -10). Body: imperative, mention the root-cause mechanism, why the test catches it. End with the standard Co-Authored-By line.

Push and open the PR with gh pr create. The PR body should include:

• Summary bullet list (what changed, what it closes)
• Why section (root cause walkthrough — the most valuable part)
• Test plan checklist with what you actually verified locally

11. Update memory

After the PR is open, write a project_issue_<N>.md memory and add a one-line entry to MEMORY.md. Capture: what was non-obvious about the bug, what diagnosis steps led to the fix, and what conventions/files the next person needs. Don't re-document the code — just the insights.

Anti-patterns to avoid

• Asking the user "is the bug visible now?" repeatedly. Capture a screenshot or read logs. If you can't decide from one capture + one question, you haven't framed the question precisely enough.
• Simplifying the repro before confirming. A "minimal" repro that doesn't trigger the bug is worse than a verbose one that does.
• Writing the test after the fix. Develop test-first (step 7 before step 8) so the failure is real and you don't have to comment out the fix to verify the test catches anything. A test authored against already-fixed code can pass for the wrong reason.
• Mixing fix + test + cleanup in one commit. Project policy is strict here — every fix needs a separately revertible test commit.
• Force-pushing for "cleanliness". Never. Stack new commits.
• Trusting memory over code. Memory files name specific functions and files; verify they still exist before recommending — projects refactor.

Tooling reference

• Issue triage: gh issue view <N> --repo Live-Charts/LiveCharts2
• Auto-launch sample: LVC_SAMPLE=<path> env var (Avalonia, WPF, WinUI, MAUI, Uno).
• In-app screenshot: LVC_SCREENSHOT=<path> env var (Avalonia, WPF, WinUI). Sample renders to PNG after LVC_SCREENSHOT_DELAY_MS (default
  3000. and exits.
• External screenshot fallback — one script per OS, all take the same <ProcessName> <OutPath> [WaitSeconds] shape:
  • Windows: .claude/scripts/capture-window.ps1 (also accepts -WindowTitle <substring> instead of -ProcessName)
  • macOS: .claude/scripts/capture-window-macos.sh
  • Linux: .claude/scripts/capture-window-linux.sh (X11 / sway / GNOME)
• Factos UI tests: tests/UITests/, set appToRun in Program.cs for local runs.
• Build per-platform: LiveCharts.<Platform>.slnx solutions.