Add baseline screenshots and visual regression testing

Add infrastructure for visual regression testing of wxWidgets WASM rendering:

- Add 121 baseline screenshots capturing all UI states tested by Playwright
- Add scripts/compare-screenshots.sh to compare test results vs baseline
- Update tests/WHATWORKS.md with documentation on screenshot comparison
- Update wxwidgets submodule with dialog refactoring

The baseline screenshots cover:
- Main app tabs (Controls, Text Input, Drawing, Lists, OpenGL, Grid, Dialogs)
- Standalone test apps (menu, toolbar, layout, aui, clipboard, filedialog, etc.)
- Dialog/MessageBox rendering (info, yes/no, error, custom dialogs)
- OpenGL rendering tests (immediate mode, vertex arrays, matrix ops)
- UI interactions (dropdowns, tab switching, z-ordering)

Use ./scripts/compare-screenshots.sh to check for visual regressions.
Small byte differences (<2%) are typically timestamps, not visual changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

scripts/compare-screenshots.sh

@@ -0,0 +1,107 @@
+#!/bin/bash
+
+# Compare test screenshots with baseline screenshots
+# This script compares all PNG files in baseline-screenshots with test-results
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+PROJECT_ROOT="$(dirname "$SCRIPT_DIR")"
+BASELINE_DIR="$PROJECT_ROOT/tests/baseline-screenshots"
+TEST_RESULTS_DIR="$PROJECT_ROOT/tests/test-results"
+
+# Check if directories exist
+if [ ! -d "$BASELINE_DIR" ]; then
+    echo "ERROR: Baseline directory not found: $BASELINE_DIR"
+    exit 1
+fi
+
+if [ ! -d "$TEST_RESULTS_DIR" ]; then
+    echo "ERROR: Test results directory not found: $TEST_RESULTS_DIR"
+    exit 1
+fi
+
+echo "=== Screenshot Comparison ==="
+echo "Baseline:     $BASELINE_DIR"
+echo "Test Results: $TEST_RESULTS_DIR"
+echo ""
+
+# Counters
+total=0
+identical=0
+different=0
+missing_current=0
+extra_current=0
+
+# Compare all baseline screenshots
+echo "=== Comparing Baseline Screenshots ==="
+for baseline in "$BASELINE_DIR"/*.png; do
+    filename=$(basename "$baseline")
+    current="$TEST_RESULTS_DIR/$filename"
+    total=$((total + 1))
+
+    if [ ! -f "$current" ]; then
+        echo "MISSING: $filename (not in test results)"
+        missing_current=$((missing_current + 1))
+        continue
+    fi
+
+    # Compare using cmp (byte-by-byte comparison)
+    if cmp -s "$baseline" "$current"; then
+        identical=$((identical + 1))
+        # Only show identical files if verbose
+        if [ "$1" = "-v" ] || [ "$1" = "--verbose" ]; then
+            echo "IDENTICAL: $filename"
+        fi
+    else
+        different=$((different + 1))
+
+        # Get file sizes
+        baseline_size=$(stat -f%z "$baseline" 2>/dev/null || stat -c%s "$baseline")
+        current_size=$(stat -f%z "$current" 2>/dev/null || stat -c%s "$current")
+        diff_bytes=$((current_size - baseline_size))
+
+        # Calculate percent difference
+        if [ "$baseline_size" -gt 0 ]; then
+            diff_pct=$(echo "scale=2; ($diff_bytes * 100) / $baseline_size" | bc)
+        else
+            diff_pct="N/A"
+        fi
+
+        echo "DIFFERENT: $filename (baseline: ${baseline_size}B, current: ${current_size}B, diff: ${diff_bytes}B / ${diff_pct}%)"
+    fi
+done
+
+# Check for extra files in test results not in baseline
+echo ""
+echo "=== Checking for Extra Screenshots ==="
+for current in "$TEST_RESULTS_DIR"/*.png; do
+    filename=$(basename "$current")
+    baseline="$BASELINE_DIR/$filename"
+
+    if [ ! -f "$baseline" ]; then
+        extra_current=$((extra_current + 1))
+        current_size=$(stat -f%z "$current" 2>/dev/null || stat -c%s "$current")
+        echo "EXTRA: $filename (${current_size}B, not in baseline)"
+    fi
+done
+
+# Summary
+echo ""
+echo "=== Summary ==="
+echo "Total baseline screenshots: $total"
+echo "Identical:                  $identical"
+echo "Different:                  $different"
+echo "Missing from test results:  $missing_current"
+echo "Extra in test results:      $extra_current"
+
+# Exit with error if there are differences
+if [ "$different" -gt 0 ] || [ "$missing_current" -gt 0 ]; then
+    echo ""
+    echo "WARNING: There are differences between baseline and test results!"
+    exit 1
+else
+    echo ""
+    echo "SUCCESS: All screenshots match baseline!"
+    exit 0
+fi

tests/WHATWORKS.md

@@ -227,6 +227,80 @@ cd tests/wasm-app && ./build-test-apps.sh
 
 ---
 
+## Baseline Screenshots
+
+The test suite captures screenshots during test runs for visual regression testing. These screenshots are compared against a baseline to detect unexpected visual changes.
+
+### Directory Structure
+
+```
+tests/
+├── baseline-screenshots/     # Known-good reference screenshots (121 files)
+├── test-results/            # Screenshots from latest test run
+└── wasm-app/
+    └── e2e/                 # Playwright test specs
+```
+
+### Comparing Screenshots
+
+Use the comparison script to check for visual regressions:
+
+```bash
+./scripts/compare-screenshots.sh
+```
+
+This will:
+- Compare all baseline screenshots with current test results
+- Report identical, different, and missing screenshots
+- Show file size differences (useful for detecting changes)
+
+Example output:
+```
+=== Screenshot Comparison ===
+Baseline:     /path/to/tests/baseline-screenshots
+Test Results: /path/to/tests/test-results
+
+=== Comparing Baseline Screenshots ===
+DIFFERENT: dialogs-msgbox-info-open.png (baseline: 47468B, current: 47478B, diff: 10B / .02%)
+
+=== Summary ===
+Total baseline screenshots: 121
+Identical:                  54
+Different:                  67
+Missing from test results:  0
+```
+
+### Understanding Differences
+
+Small byte differences (<2%) are typically caused by:
+- **Timestamps** in event logs (e.g., `[19:45:35]` vs `[18:49:35]`)
+- **PNG compression** variations between runs
+- **Anti-aliasing** differences
+
+These are **not** visual regressions. Visually inspect screenshots if you see larger differences.
+
+### Updating Baseline Screenshots
+
+After verifying changes are intentional, update the baseline:
+
+```bash
+# Copy current screenshots to baseline
+cp tests/test-results/*.png tests/baseline-screenshots/
+
+# Or selectively update specific screenshots
+cp tests/test-results/dialog-*.png tests/baseline-screenshots/
+```
+
+### Running Tests with Screenshots
+
+```bash
+cd tests/wasm-app
+npm test                    # Run all tests (saves screenshots to test-results/)
+npx playwright test --ui    # Interactive mode with screenshot preview
+```
+
+---
+
 ## Screenshots Reference
 
 | Screenshot | Shows |