feat(preview-server): dark mode switcher

.changeset/flat-masks-take.md

@@ -0,0 +1,6 @@
+---
+"@react-email/preview-server": minor
+"react-email": minor
+---
+
+Dark mode switcher emulating email client color inversion

packages/preview-server/package.json

@@ -23,6 +23,7 @@
     "@radix-ui/react-popover": "1.1.15",
     "@radix-ui/react-slot": "1.2.3",
     "@radix-ui/react-tabs": "1.1.13",
+    "@radix-ui/react-toggle": "1.1.10",
     "@radix-ui/react-toggle-group": "1.1.11",
     "@radix-ui/react-tooltip": "1.2.8",
     "@react-email/tailwind": "workspace:2.0.0-canary.1",
@@ -33,6 +34,7 @@
     "@types/webpack": "5.28.5",
     "autoprefixer": "10.4.21",
     "clsx": "2.1.1",
+    "colorjs.io": "0.5.2",
     "esbuild": "0.25.10",
     "framer-motion": "12.23.22",
     "json5": "2.2.3",

packages/preview-server/src/app/preview/[...slug]/email-frame.tsx

@@ -0,0 +1,306 @@
+import { Slot } from '@radix-ui/react-slot';
+import Color from 'colorjs.io';
+import type { ComponentProps } from 'react';
+
+function* walkDom(element: Element): Generator<Element> {
+  if (element.children.length > 0) {
+    for (let i = 0; i < element.children.length; i++) {
+      const child = element.children.item(i)!;
+      yield child;
+      yield* walkDom(child);
+    }
+  }
+}
+
+function invertColor(colorString: string, mode: 'foreground' | 'background') {
+  try {
+    const color = new Color(colorString).to('lch');
+
+    if (mode === 'background') {
+      // Keeps the same lightness if it's already dark. If it's bright inverts the lightness
+      // - This is a characteristic from Outlook iOS
+      // - Parcel does something very similar
+      //
+      // The 0.75 factor ensures that, even if the lightness is 100%, the final inverted is going to be 25%
+      // - This is a characteristic from Apple Mail
+      //
+      // The two extra 50 terms are so that the lightness inversion doesn't become a step function
+      if (color.lch.l! >= 50) {
+        color.lch.l = 50 - (color.lch.l! - 50) * 0.75;
+      }
+    } else if (mode === 'foreground') {
+      // The same as what's done for background, but inverts the check for brightness.
+      // If the color is already bright, then it keeps the same. If the color is dark, then it inverts the brightness
+      if (color.lch.l! < 50) {
+        color.lch.l = 50 - (color.lch.l! - 50) * 0.75;
+      }
+    }
+
+    // While not exactly, I've found that email clients generally tend to reduce the chrome by 20%.
+    // Apple Mail specifically reduces by exactly 20%, so we're closer to Apple Mail in this sense as well.
+    color.lch.c! *= 0.8;
+
+    return color.toString();
+  } catch (exception) {
+    console.error(`couldn't invert color ${colorString}`, exception);
+    return colorString;
+  }
+}
+
+const colorRegex = () =>
+  /#[0-9a-f]{3,4}|#[0-9a-f]{6,8}|(rgb|rgba|hsl|hsv|oklab|oklch|lab|lch|hwb)\s*\(.*?\)/gi;
+const namedColors = {
+  aliceblue: '#f0f8ff',
+  antiquewhite: '#faebd7',
+  aqua: '#00ffff',
+  aquamarine: '#7fffd4',
+  azure: '#f0ffff',
+  beige: '#f5f5dc',
+  bisque: '#ffe4c4',
+  black: '#000000',
+  blanchedalmond: '#ffebcd',
+  blue: '#0000ff',
+  blueviolet: '#8a2be2',
+  brown: '#a52a2a',
+  burlywood: '#deb887',
+  cadetblue: '#5f9ea0',
+  chartreuse: '#7fff00',
+  chocolate: '#d2691e',
+  coral: '#ff7f50',
+  cornflowerblue: '#6495ed',
+  cornsilk: '#fff8dc',
+  crimson: '#dc143c',
+  cyan: '#00ffff',
+  darkblue: '#00008b',
+  darkcyan: '#008b8b',
+  darkgoldenrod: '#b8860b',
+  darkgray: '#a9a9a9',
+  darkgreen: '#006400',
+  darkgrey: '#a9a9a9',
+  darkkhaki: '#bdb76b',
+  darkmagenta: '#8b008b',
+  darkolivegreen: '#556b2f',
+  darkorange: '#ff8c00',
+  darkorchid: '#9932cc',
+  darkred: '#8b0000',
+  darksalmon: '#e9967a',
+  darkseagreen: '#8fbc8f',
+  darkslateblue: '#483d8b',
+  darkslategray: '#2f4f4f',
+  darkslategrey: '#2f4f4f',
+  darkturquoise: '#00ced1',
+  darkviolet: '#9400d3',
+  deeppink: '#ff1493',
+  deepskyblue: '#00bfff',
+  dimgray: '#696969',
+  dimgrey: '#696969',
+  dodgerblue: '#1e90ff',
+  firebrick: '#b22222',
+  floralwhite: '#fffaf0',
+  forestgreen: '#228b22',
+  fuchsia: '#ff00ff',
+  gainsboro: '#dcdcdc',
+  ghostwhite: '#f8f8ff',
+  gold: '#ffd700',
+  goldenrod: '#daa520',
+  gray: '#808080',
+  green: '#008000',
+  greenyellow: '#adff2f',
+  grey: '#808080',
+  honeydew: '#f0fff0',
+  hotpink: '#ff69b4',
+  indianred: '#cd5c5c',
+  indigo: '#4b0082',
+  ivory: '#fffff0',
+  khaki: '#f0e68c',
+  lavender: '#e6e6fa',
+  lavenderblush: '#fff0f5',
+  lawngreen: '#7cfc00',
+  lemonchiffon: '#fffacd',
+  lightblue: '#add8e6',
+  lightcoral: '#f08080',
+  lightcyan: '#e0ffff',
+  lightgoldenrodyellow: '#fafad2',
+  lightgray: '#d3d3d3',
+  lightgreen: '#90ee90',
+  lightgrey: '#d3d3d3',
+  lightpink: '#ffb6c1',
+  lightsalmon: '#ffa07a',
+  lightseagreen: '#20b2aa',
+  lightskyblue: '#87cefa',
+  lightslategray: '#778899',
+  lightslategrey: '#778899',
+  lightsteelblue: '#b0c4de',
+  lightyellow: '#ffffe0',
+  lime: '#00ff00',
+  limegreen: '#32cd32',
+  linen: '#faf0e6',
+  magenta: '#ff00ff',
+  maroon: '#800000',
+  mediumaquamarine: '#66cdaa',
+  mediumblue: '#0000cd',
+  mediumorchid: '#ba55d3',
+  mediumpurple: '#9370db',
+  mediumseagreen: '#3cb371',
+  mediumslateblue: '#7b68ee',
+  mediumspringgreen: '#00fa9a',
+  mediumturquoise: '#48d1cc',
+  mediumvioletred: '#c71585',
+  midnightblue: '#191970',
+  mintcream: '#f5fffa',
+  mistyrose: '#ffe4e1',
+  moccasin: '#ffe4b5',
+  navajowhite: '#ffdead',
+  navy: '#000080',
+  oldlace: '#fdf5e6',
+  olive: '#808000',
+  olivedrab: '#6b8e23',
+  orange: '#ffa500',
+  orangered: '#ff4500',
+  orchid: '#da70d6',
+  palegoldenrod: '#eee8aa',
+  palegreen: '#98fb98',
+  paleturquoise: '#afeeee',
+  palevioletred: '#db7093',
+  papayawhip: '#ffefd5',
+  peachpuff: '#ffdab9',
+  peru: '#cd853f',
+  pink: '#ffc0cb',
+  plum: '#dda0dd',
+  powderblue: '#b0e0e6',
+  purple: '#800080',
+  rebeccapurple: '#663399',
+  red: '#ff0000',
+  rosybrown: '#bc8f8f',
+  royalblue: '#4169e1',
+  saddlebrown: '#8b4513',
+  salmon: '#fa8072',
+  sandybrown: '#f4a460',
+  seagreen: '#2e8b57',
+  seashell: '#fff5ee',
+  sienna: '#a0522d',
+  silver: '#c0c0c0',
+  skyblue: '#87ceeb',
+  slateblue: '#6a5acd',
+  slategray: '#708090',
+  slategrey: '#708090',
+  snow: '#fffafa',
+  springgreen: '#00ff7f',
+  steelblue: '#4682b4',
+  tan: '#d2b48c',
+  teal: '#008080',
+  thistle: '#d8bfd8',
+  tomato: '#ff6347',
+  transparent: 'rgba(0,0,0,0)',
+  turquoise: '#40e0d0',
+  violet: '#ee82ee',
+  wheat: '#f5deb3',
+  white: '#ffffff',
+  whitesmoke: '#f5f5f5',
+  yellow: '#ffff00',
+  yellowgreen: '#9acd32',
+};
+const namedColorRegex = new RegExp(
+  `${Object.keys(namedColors).join('|')}`,
+  'gi',
+);
+
+function applyColorInversion(iframe: HTMLIFrameElement) {
+  const { contentDocument, contentWindow } = iframe;
+  if (!contentDocument || !contentWindow) return;
+
+  if (!contentDocument.body.style.color) {
+    contentDocument.body.style.color = 'rgb(0, 0, 0)';
+  }
+
+  for (const element of walkDom(contentDocument.documentElement)) {
+    if (
+      element instanceof
+      (contentWindow as unknown as typeof globalThis).HTMLElement
+    ) {
+      if (element.style.color) {
+        element.style.color = element.style.color
+          .replaceAll(colorRegex(), (color) => invertColor(color, 'foreground'))
+          .replaceAll(namedColorRegex, (namedColor) =>
+            invertColor(namedColors[namedColor], 'foreground'),
+          );
+        namedColorRegex.lastIndex = 0;
+      }
+      if (element.style.background) {
+        element.style.background = element.style.background
+          .replaceAll(colorRegex(), (color) => invertColor(color, 'background'))
+          .replaceAll(namedColorRegex, (namedColor) =>
+            invertColor(namedColors[namedColor], 'foreground'),
+          );
+        namedColorRegex.lastIndex = 0;
+      }
+      if (element.style.backgroundColor) {
+        element.style.backgroundColor = element.style.backgroundColor
+          .replaceAll(colorRegex(), (color) => invertColor(color, 'background'))
+          .replaceAll(namedColorRegex, (namedColor) =>
+            invertColor(namedColors[namedColor], 'foreground'),
+          );
+        namedColorRegex.lastIndex = 0;
+      }
+      if (element.style.borderColor) {
+        element.style.borderColor = element.style.borderColor
+          .replaceAll(colorRegex(), (color) => invertColor(color, 'background'))
+          .replaceAll(namedColorRegex, (namedColor) =>
+            invertColor(namedColors[namedColor], 'foreground'),
+          );
+        namedColorRegex.lastIndex = 0;
+      }
+      if (element.style.border) {
+        element.style.border = element.style.border
+          .replaceAll(colorRegex(), (color) => invertColor(color, 'background'))
+          .replaceAll(namedColorRegex, (namedColor) =>
+            invertColor(namedColors[namedColor], 'foreground'),
+          );
+        namedColorRegex.lastIndex = 0;
+      }
+    }
+  }
+}
+
+interface EmailFrameProps extends ComponentProps<'iframe'> {
+  markup: string;
+  width: number;
+  height: number;
+  darkMode: boolean;
+}
+
+export function EmailFrame({
+  markup,
+  width,
+  height,
+  darkMode,
+  ...rest
+}: EmailFrameProps) {
+  return (
+    <Slot
+      ref={(iframe: HTMLIFrameElement) => {
+        if (!iframe) return;
+
+        if (darkMode) {
+          applyColorInversion(iframe);
+        }
+      }}
+    >
+      <iframe
+        srcDoc={markup}
+        width={width}
+        height={height}
+        onLoad={(event) => {
+          if (darkMode) {
+            const iframe = event.currentTarget;
+            applyColorInversion(iframe);
+          }
+        }}
+        {...rest}
+        // This key makes sure that the iframe itself remounts to the DOM when theme changes, so
+        // that the color changes in dark mode can be easily undone when switching to light mode.
+        key={darkMode ? 'iframe-inverted-colors' : 'iframe-normal-colors'}
+      />
+    </Slot>
+  );
+}