fix(accessibility): reconcile robust focus appearance after #270

scripts/test_viewer_focus_appearance.py

@@ -0,0 +1,200 @@
+#!/usr/bin/env python3
+"""Verify that the viewer focus indicator stays visible on light and dark surfaces."""
+
+from __future__ import annotations
+
+import re
+import unittest
+from dataclasses import dataclass
+from pathlib import Path
+
+
+REPOSITORY_ROOT = Path(__file__).resolve().parents[1]
+VIEWER_CSS = REPOSITORY_ROOT / "src/main/resources/static/assets/viewer/viewer.css"
+MINIMUM_CONTRAST = 3.0
+MINIMUM_BAND_WIDTH_PX = 2.0
+
+
+@dataclass(frozen=True)
+class FocusBand:
+    """Describe one solid, independently visible focus-indicator band."""
+
+    width_px: float
+    color_hex: str
+
+
+def _srgb_channel_to_linear(channel: float) -> float:
+    """Convert one normalized sRGB channel to linear-light space."""
+
+    if channel <= 0.04045:
+        return channel / 12.92
+    return ((channel + 0.055) / 1.055) ** 2.4
+
+
+def _relative_luminance(color_hex: str) -> float:
+    """Return WCAG relative luminance for one six-digit hexadecimal color."""
+
+    match = re.fullmatch(r"#([0-9a-fA-F]{6})", color_hex)
+    if match is None:
+        raise AssertionError(f"unsupported focus color: {color_hex}")
+    digits = match.group(1)
+    red, green, blue = (
+        int(digits[index : index + 2], 16) / 255.0 for index in (0, 2, 4)
+    )
+    linear_red, linear_green, linear_blue = (
+        _srgb_channel_to_linear(channel) for channel in (red, green, blue)
+    )
+    return 0.2126 * linear_red + 0.7152 * linear_green + 0.0722 * linear_blue
+
+
+def _contrast_ratio(first_hex: str, second_hex: str) -> float:
+    """Return WCAG contrast ratio for two six-digit hexadecimal colors."""
+
+    first = _relative_luminance(first_hex)
+    second = _relative_luminance(second_hex)
+    lighter = max(first, second)
+    darker = min(first, second)
+    return (lighter + 0.05) / (darker + 0.05)
+
+
+def _mix_with_white(color_hex: str, white_percentage: float) -> str:
+    """Resolve the viewer's sRGB color-mix form into one hexadecimal color."""
+
+    match = re.fullmatch(r"#([0-9a-fA-F]{6})", color_hex)
+    if match is None:
+        raise AssertionError(f"unsupported mix color: {color_hex}")
+    weight = white_percentage / 100.0
+    digits = match.group(1)
+    channels = [int(digits[index : index + 2], 16) for index in (0, 2, 4)]
+    mixed = [round(channel * (1.0 - weight) + 255 * weight) for channel in channels]
+    return "#" + "".join(f"{channel:02x}" for channel in mixed)
+
+
+def _custom_property(css: str, property_name: str) -> str:
+    """Return one root custom-property value from the viewer stylesheet."""
+
+    match = re.search(rf"{re.escape(property_name)}\s*:\s*([^;]+);", css)
+    if match is None:
+        raise AssertionError(f"missing CSS property: {property_name}")
+    return match.group(1).strip()
+
+
+def _focus_block(css: str) -> str:
+    """Return the top-level focus-visible declaration block."""
+
+    match = re.search(r":focus-visible\s*\{(?P<body>.*?)\}", css, re.DOTALL)
+    if match is None:
+        raise AssertionError("missing :focus-visible block")
+    return match.group("body")
+
+
+def _resolve_focus_color(css: str, expression: str) -> str:
+    """Resolve the limited color syntax used by the focus contract."""
+
+    value = expression.strip()
+    variable_match = re.fullmatch(r"var\((--[a-zA-Z0-9_-]+)\)", value)
+    if variable_match is not None:
+        return _custom_property(css, variable_match.group(1))
+    mix_match = re.fullmatch(
+        r"color-mix\(in\s+srgb,\s*var\((--[a-zA-Z0-9_-]+)\),\s*#ffffff\s+([0-9.]+)%\)",
+        value,
+        re.IGNORECASE,
+    )
+    if mix_match is not None:
+        base_color = _custom_property(css, mix_match.group(1))
+        return _mix_with_white(base_color, float(mix_match.group(2)))
+    if re.fullmatch(r"#[0-9a-fA-F]{6}", value):
+        return value.lower()
+    raise AssertionError(f"unsupported focus color expression: {value}")
+
+
+def _outline_offset_px(block: str) -> float:
+    """Return the non-negative outline offset that exposes an inner shadow band."""
+
+    match = re.search(
+        r"outline-offset\s*:\s*([+-]?[0-9.]+)px\s*;",
+        block,
+        re.IGNORECASE,
+    )
+    if match is None:
+        raise AssertionError("focus-visible must define an outline-offset")
+    offset = float(match.group(1))
+    if offset < 0.0:
+        raise AssertionError("focus-visible outline-offset must not be negative")
+    return offset
+
+
+def _focus_bands(css: str) -> list[FocusBand]:
+    """Return non-overlapping solid bands visible in the focus-visible rule."""
+
+    block = _focus_block(css)
+    outlines = re.findall(
+        r"outline\s*:\s*([0-9.]+)px\s+solid\s+([^;]+);",
+        block,
+        re.IGNORECASE,
+    )
+    if not outlines:
+        raise AssertionError("focus-visible must define a solid outline")
+    outline_width, outline_expression = outlines[-1]
+    bands = [
+        FocusBand(float(outline_width), _resolve_focus_color(css, outline_expression))
+    ]
+
+    shadow_match = re.search(
+        r"box-shadow\s*:\s*0\s+0\s+0\s+([0-9.]+)px\s+(#[0-9a-fA-F]{6})\s*;",
+        block,
+        re.IGNORECASE,
+    )
+    if shadow_match is not None:
+        shadow_spread = float(shadow_match.group(1))
+        exposed_shadow_width = min(shadow_spread, _outline_offset_px(block))
+        if exposed_shadow_width > 0.0:
+            bands.append(FocusBand(exposed_shadow_width, shadow_match.group(2).lower()))
+    return bands
+
+
+class ViewerFocusAppearanceTest(unittest.TestCase):
+    """Keep keyboard focus visible across light and dark viewer surfaces."""
+
+    def test_focus_indicator_has_qualifying_band_on_light_and_dark_backgrounds(self) -> None:
+        """Require a >=2px, >=3:1 band for both white and black backgrounds."""
+
+        css = VIEWER_CSS.read_text(encoding="utf-8")
+        bands = _focus_bands(css)
+
+        for background in ("#ffffff", "#000000"):
+            with self.subTest(background=background):
+                qualifying_width = sum(
+                    band.width_px
+                    for band in bands
+                    if _contrast_ratio(band.color_hex, background) >= MINIMUM_CONTRAST
+                )
+                self.assertGreaterEqual(
+                    qualifying_width,
+                    MINIMUM_BAND_WIDTH_PX,
+                    (
+                        f"focus indicator needs at least {MINIMUM_BAND_WIDTH_PX:g}px "
+                        f"at {MINIMUM_CONTRAST:g}:1 against {background}; "
+                        f"bands={bands}"
+                    ),
+                )
+
+    def test_focus_band_parser_does_not_double_count_overlapping_shadow(self) -> None:
+        """Count only shadow pixels exposed before an overlapping outline begins."""
+
+        css = """
+        :focus-visible {
+          outline: 3px solid #000000;
+          outline-offset: 1px;
+          box-shadow: 0 0 0 3px #ffffff;
+        }
+        """
+
+        self.assertEqual(
+            [FocusBand(3.0, "#000000"), FocusBand(1.0, "#ffffff")],
+            _focus_bands(css),
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()

src/main/resources/static/assets/viewer/viewer.css

@@ -44,9 +44,9 @@ a:hover {
 }
 
 :focus-visible {
-  outline: 3px solid var(--focus);
-  outline: 3px solid color-mix(in srgb, var(--focus), #ffffff 25%);
+  outline: 3px solid var(--ink);
   outline-offset: 3px;
+  box-shadow: 0 0 0 3px #ffffff;
 }
 
 .skip-link {