Excessibility Debug - Timeline & State Analysis

Debug LiveView issues by inspecting state evolution and rendered HTML.

Tools

• Timeline - Shows state at mount, handle_event, render
• Snapshots - Captured HTML at specific moments
• Analyzers - Memory, hypothesis, code pointers

Four-Phase Debug Process

Phase 1: Capture with Snapshots

Add temporary html_snapshot(view) calls around the problem area:

test "debugging problematic feature" do
  {:ok, view, _html} = live(conn, "/page")

  html_snapshot(view)  # before the problem

  view |> element("button") |> render_click()

  html_snapshot(view)  # after - what changed?
end

Run with telemetry capture:

mix excessibility.debug test/failing_test.exs

Phase 2: Analyze Timeline

Read test/excessibility/timeline.json:

{
  "test": "test debugging problematic feature",
  "duration_ms": 142,
  "timeline": [
    {
      "sequence": 1,
      "event": "mount",
      "timestamp": "2024-01-15T10:30:00Z",
      "memory_size": 1024,
      "key_state": {"user": null, "items": []},
      "changes": {}
    },
    {
      "sequence": 2,
      "event": "handle_event:click",
      "timestamp": "2024-01-15T10:30:00.050Z",
      "memory_size": 4096,
      "key_state": {"user": null, "items": ["a", "b", "c"]},
      "changes": {"items": "changed"}
    }
  ]
}

Look for:

• Assigns at each event
• What changed between renders
• Unexpected state

Phase 3: Inspect Snapshots

Read captured HTML files in test/excessibility/html_snapshots/:

# List all snapshots
ls test/excessibility/html_snapshots/

# Open in browser
open test/excessibility/html_snapshots/MyModule_42.html

Check for:

• Missing elements
• Wrong content
• Incorrect attributes
• Compare before/after

Phase 4: Fix

1. Use hypothesis analyzer for root cause suggestions
2. Add test for the specific issue
3. Implement fix
4. Verify with mix excessibility
5. Remove temporary snapshots

Commands

# Basic debug run
mix excessibility.debug test/my_test.exs

# With specific analyzers
mix excessibility.debug test/my_test.exs --analyze=memory,hypothesis

# Verbose output (detailed stats)
mix excessibility.debug test/my_test.exs --verbose

# Full assigns (no filtering)
mix excessibility.debug test/my_test.exs --full

# Highlight specific assigns
mix excessibility.debug test/my_test.exs --highlight=user,cart,items

Available Analyzers

Analyzer	What it finds
memory	Memory bloat, leaks (default, enabled)
hypothesis	Suggested root causes
code_pointer	Source locations for issues
accessibility_correlation	State changes affecting a11y

Accessibility Debugging

For axe-core/WCAG issues specifically:

mix excessibility.debug test/file.exs --analyze=accessibility_correlation

This correlates:

• State changes with a11y violations
• Which events caused accessible name loss
• Missing ARIA attributes after state changes

Reading the Timeline

Memory Growth

{
  "sequence": 5,
  "event": "handle_event:load_more",
  "memory_size": 102400,  // 100KB - growing!
  "key_state": {"items": "[1000 items]"}
}

Red flag: Memory growing across events suggests unbounded list accumulation.

State Changes

{
  "changes": {
    "user": "changed",        // user assign was modified
    "form": "added",          // new assign appeared
    "error_message": "removed" // assign was deleted
  }
}

Event Flow

Normal flow: mount → handle_params → handle_event:* → render

Suspicious patterns:

• Many render events without handle_event (component re-rendering)
• handle_event without state change (noop handlers)
• Missing handle_params after navigation

Debugging Patterns

State Not Updating

1. Check timeline for handle_event
2. Look at changes - is the assign modified?
3. If no change, the handler might not be updating socket

# Problem: No change in timeline
def handle_event("click", _params, socket) do
  # Forgot to assign!
  {:noreply, socket}
end

# Fix: Assign the change
def handle_event("click", _params, socket) do
  {:noreply, assign(socket, :clicked, true)}
end

Wrong Content Rendered

1. Snapshot shows wrong HTML
2. Check timeline state at that point
3. State correct but HTML wrong? Check template
4. State wrong? Trace back through events

Memory Leak

1. Run with --analyze=memory
2. Check for growing memory_size across events
3. Look for lists that accumulate
4. Use --highlight=suspects to focus on likely culprits

Integration with superpowers

This skill works well with:

• systematic-debugging - Four-phase framework for root cause
• root-cause-tracing - Trace back through call stack
• e11y-tdd - Fix with TDD after finding cause