Bug 41916: Letterboxing preferences UI

browser/components/preferences/jar.mn

@@ -22,3 +22,10 @@ browser.jar:
    content/browser/preferences/more-from-mozilla-qr-code-simple-cn.svg
    content/browser/preferences/web-appearance-dark.svg
    content/browser/preferences/web-appearance-light.svg
+
+   content/browser/preferences/letterboxing.js
+   content/browser/preferences/letterboxing.css
+   content/browser/preferences/letterboxing-middle-dark.svg
+   content/browser/preferences/letterboxing-middle-light.svg
+   content/browser/preferences/letterboxing-top-dark.svg
+   content/browser/preferences/letterboxing-top-light.svg

browser/components/preferences/letterboxing-middle-dark.svg

+<svg width="62" height="50" viewBox="0 0 62 50" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g filter="url(#filter0_d_2272_529)">
+<g clip-path="url(#clip0_2272_529)">
+<path d="M58 3H4V45H58V3Z" fill="#42414D"/>
+<path d="M58 3H4V14H58V3Z" fill="#42414D"/>
+<path d="M58 3H4V8H58V3Z" fill="#2B2A33"/>
+<path d="M20 5H10C9.44772 5 9 5.44772 9 6C9 6.55228 9.44772 7 10 7H20C20.5523 7 21 6.55228 21 6C21 5.44772 20.5523 5 20 5Z" fill="white"/>
+<path d="M56 11C56 10.4477 55.5523 10 55 10C54.4477 10 54 10.4477 54 11C54 11.5523 54.4477 12 55 12C55.5523 12 56 11.5523 56 11Z" fill="white"/>
+<path d="M8 11C8 10.4477 7.55228 10 7 10C6.44772 10 6 10.4477 6 11C6 11.5523 6.44772 12 7 12C7.55228 12 8 11.5523 8 11Z" fill="white"/>
+<path d="M12 11C12 10.4477 11.5523 10 11 10C10.4477 10 10 10.4477 10 11C10 11.5523 10.4477 12 11 12C11.5523 12 12 11.5523 12 11Z" fill="white"/>
+<path d="M51 10H15C14.4477 10 14 10.4477 14 11C14 11.5523 14.4477 12 15 12H51C51.5523 12 52 11.5523 52 11C52 10.4477 51.5523 10 51 10Z" fill="white"/>
+<path d="M58 14H4V45H58V14Z" fill="#5B5B66"/>
+<rect x="8" y="18" width="46" height="23" rx="2" fill="#3A3944"/>
+<path d="M38 22H13C12.4477 22 12 22.4477 12 23C12 23.5523 12.4477 24 13 24H38C38.5523 24 39 23.5523 39 23C39 22.4477 38.5523 22 38 22Z" fill="white"/>
+<path d="M27 26H13C12.4477 26 12 26.4477 12 27C12 27.5523 12.4477 28 13 28H27C27.5523 28 28 27.5523 28 27C28 26.4477 27.5523 26 27 26Z" fill="white"/>
+<path d="M42 30H13C12.4477 30 12 30.4477 12 31C12 31.5523 12.4477 32 13 32H42C42.5523 32 43 31.5523 43 31C43 30.4477 42.5523 30 42 30Z" fill="white"/>
+</g>
+<rect x="4.5" y="3.5" width="53" height="41" rx="1.5" stroke="#8F8F9D" shape-rendering="crispEdges"/>
+</g>
+<defs>
+<filter id="filter0_d_2272_529" x="0" y="0" width="62" height="50" filterUnits="userSpaceOnUse" color-interpolation-filters="sRGB">
+<feFlood flood-opacity="0" result="BackgroundImageFix"/>
+<feColorMatrix in="SourceAlpha" type="matrix" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 127 0" result="hardAlpha"/>
+<feOffset dy="1"/>
+<feGaussianBlur stdDeviation="2"/>
+<feComposite in2="hardAlpha" operator="out"/>
+<feColorMatrix type="matrix" values="0 0 0 0 0.0470588 0 0 0 0 0.0470588 0 0 0 0 0.0509804 0 0 0 0.1 0"/>
+<feBlend mode="normal" in2="BackgroundImageFix" result="effect1_dropShadow_2272_529"/>
+<feBlend mode="normal" in="SourceGraphic" in2="effect1_dropShadow_2272_529" result="shape"/>
+</filter>
+<clipPath id="clip0_2272_529">
+<rect x="4" y="3" width="54" height="42" rx="2" fill="white"/>
+</clipPath>
+</defs>
+</svg>

browser/components/preferences/letterboxing-middle-light.svg

+<svg width="62" height="50" viewBox="0 0 62 50" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g filter="url(#filter0_d_202_960)">
+<g clip-path="url(#clip0_202_960)">
+<path d="M58 3H4V45H58V3Z" fill="#F0F0F4"/>
+<path d="M58 3H4V14H58V3Z" fill="#F0F0F4"/>
+<path d="M58 3H4V8H58V3Z" fill="#E0E0E6"/>
+<path d="M20 5H10C9.44772 5 9 5.44772 9 6C9 6.55228 9.44772 7 10 7H20C20.5523 7 21 6.55228 21 6C21 5.44772 20.5523 5 20 5Z" fill="#52525E"/>
+<path d="M56 11C56 10.4477 55.5523 10 55 10C54.4477 10 54 10.4477 54 11C54 11.5523 54.4477 12 55 12C55.5523 12 56 11.5523 56 11Z" fill="#52525E"/>
+<path d="M8 11C8 10.4477 7.55228 10 7 10C6.44772 10 6 10.4477 6 11C6 11.5523 6.44772 12 7 12C7.55228 12 8 11.5523 8 11Z" fill="#52525E"/>
+<path d="M12 11C12 10.4477 11.5523 10 11 10C10.4477 10 10 10.4477 10 11C10 11.5523 10.4477 12 11 12C11.5523 12 12 11.5523 12 11Z" fill="#52525E"/>
+<path d="M51 10H15C14.4477 10 14 10.4477 14 11C14 11.5523 14.4477 12 15 12H51C51.5523 12 52 11.5523 52 11C52 10.4477 51.5523 10 51 10Z" fill="#52525E"/>
+<path d="M58 14H4V45H58V14Z" fill="#CFCFD8"/>
+<rect x="8" y="18" width="46" height="23" rx="2" fill="white"/>
+<path d="M38 22H13C12.4477 22 12 22.4477 12 23C12 23.5523 12.4477 24 13 24H38C38.5523 24 39 23.5523 39 23C39 22.4477 38.5523 22 38 22Z" fill="#52525E"/>
+<path d="M27 26H13C12.4477 26 12 26.4477 12 27C12 27.5523 12.4477 28 13 28H27C27.5523 28 28 27.5523 28 27C28 26.4477 27.5523 26 27 26Z" fill="#52525E"/>
+<path d="M42 30H13C12.4477 30 12 30.4477 12 31C12 31.5523 12.4477 32 13 32H42C42.5523 32 43 31.5523 43 31C43 30.4477 42.5523 30 42 30Z" fill="#52525E"/>
+</g>
+<rect x="4.5" y="3.5" width="53" height="41" rx="1.5" stroke="#8F8F9D" shape-rendering="crispEdges"/>
+</g>
+<defs>
+<filter id="filter0_d_202_960" x="0" y="0" width="62" height="50" filterUnits="userSpaceOnUse" color-interpolation-filters="sRGB">
+<feFlood flood-opacity="0" result="BackgroundImageFix"/>
+<feColorMatrix in="SourceAlpha" type="matrix" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 127 0" result="hardAlpha"/>
+<feOffset dy="1"/>
+<feGaussianBlur stdDeviation="2"/>
+<feComposite in2="hardAlpha" operator="out"/>
+<feColorMatrix type="matrix" values="0 0 0 0 0.0470588 0 0 0 0 0.0470588 0 0 0 0 0.0509804 0 0 0 0.1 0"/>
+<feBlend mode="normal" in2="BackgroundImageFix" result="effect1_dropShadow_202_960"/>
+<feBlend mode="normal" in="SourceGraphic" in2="effect1_dropShadow_202_960" result="shape"/>
+</filter>
+<clipPath id="clip0_202_960">
+<rect x="4" y="3" width="54" height="42" rx="2" fill="white"/>
+</clipPath>
+</defs>
+</svg>

browser/components/preferences/letterboxing-top-dark.svg

+<svg width="62" height="50" viewBox="0 0 62 50" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g filter="url(#filter0_d_2272_508)">
+<g clip-path="url(#clip0_2272_508)">
+<path d="M58 3H4V45H58V3Z" fill="#52525E"/>
+<path d="M58 3H4V14H58V3Z" fill="#42414D"/>
+<path d="M58 3H4V8H58V3Z" fill="#2B2A33"/>
+<path d="M20 5H10C9.44772 5 9 5.44772 9 6C9 6.55228 9.44772 7 10 7H20C20.5523 7 21 6.55228 21 6C21 5.44772 20.5523 5 20 5Z" fill="white"/>
+<path d="M56 11C56 10.4477 55.5523 10 55 10C54.4477 10 54 10.4477 54 11C54 11.5523 54.4477 12 55 12C55.5523 12 56 11.5523 56 11Z" fill="white"/>
+<path d="M8 11C8 10.4477 7.55228 10 7 10C6.44772 10 6 10.4477 6 11C6 11.5523 6.44772 12 7 12C7.55228 12 8 11.5523 8 11Z" fill="white"/>
+<path d="M12 11C12 10.4477 11.5523 10 11 10C10.4477 10 10 10.4477 10 11C10 11.5523 10.4477 12 11 12C11.5523 12 12 11.5523 12 11Z" fill="white"/>
+<path d="M51 10H15C14.4477 10 14 10.4477 14 11C14 11.5523 14.4477 12 15 12H51C51.5523 12 52 11.5523 52 11C52 10.4477 51.5523 10 51 10Z" fill="white"/>
+<path d="M58 14H4V45H58V14Z" fill="#5B5B66"/>
+<path d="M8 14H55V39C55 40.1046 54.1046 41 53 41H10C8.89543 41 8 40.1046 8 39V14Z" fill="#3A3944"/>
+<path d="M38 18H13C12.4477 18 12 18.4477 12 19C12 19.5523 12.4477 20 13 20H38C38.5523 20 39 19.5523 39 19C39 18.4477 38.5523 18 38 18Z" fill="white"/>
+<path d="M27 22H13C12.4477 22 12 22.4477 12 23C12 23.5523 12.4477 24 13 24H27C27.5523 24 28 23.5523 28 23C28 22.4477 27.5523 22 27 22Z" fill="white"/>
+<path d="M42 26H13C12.4477 26 12 26.4477 12 27C12 27.5523 12.4477 28 13 28H42C42.5523 28 43 27.5523 43 27C43 26.4477 42.5523 26 42 26Z" fill="white"/>
+</g>
+<rect x="4.5" y="3.5" width="53" height="41" rx="3.5" stroke="#8F8F9D" shape-rendering="crispEdges"/>
+</g>
+<defs>
+<filter id="filter0_d_2272_508" x="0" y="0" width="62" height="50" filterUnits="userSpaceOnUse" color-interpolation-filters="sRGB">
+<feFlood flood-opacity="0" result="BackgroundImageFix"/>
+<feColorMatrix in="SourceAlpha" type="matrix" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 127 0" result="hardAlpha"/>
+<feOffset dy="1"/>
+<feGaussianBlur stdDeviation="2"/>
+<feComposite in2="hardAlpha" operator="out"/>
+<feColorMatrix type="matrix" values="0 0 0 0 0.0470588 0 0 0 0 0.0470588 0 0 0 0 0.0509804 0 0 0 0.1 0"/>
+<feBlend mode="normal" in2="BackgroundImageFix" result="effect1_dropShadow_2272_508"/>
+<feBlend mode="normal" in="SourceGraphic" in2="effect1_dropShadow_2272_508" result="shape"/>
+</filter>
+<clipPath id="clip0_2272_508">
+<rect x="4" y="3" width="54" height="42" rx="4" fill="white"/>
+</clipPath>
+</defs>
+</svg>

browser/components/preferences/letterboxing-top-light.svg

+<svg width="62" height="50" viewBox="0 0 62 50" fill="none" xmlns="http://www.w3.org/2000/svg">
+<g filter="url(#filter0_d_202_926)">
+<g clip-path="url(#clip0_202_926)">
+<path d="M58 3H4V45H58V3Z" fill="#F0F0F4"/>
+<path d="M58 3H4V14H58V3Z" fill="#F0F0F4"/>
+<path d="M58 3H4V8H58V3Z" fill="#E0E0E6"/>
+<path d="M20 5H10C9.44772 5 9 5.44772 9 6C9 6.55228 9.44772 7 10 7H20C20.5523 7 21 6.55228 21 6C21 5.44772 20.5523 5 20 5Z" fill="#52525E"/>
+<path d="M56 11C56 10.4477 55.5523 10 55 10C54.4477 10 54 10.4477 54 11C54 11.5523 54.4477 12 55 12C55.5523 12 56 11.5523 56 11Z" fill="#52525E"/>
+<path d="M8 11C8 10.4477 7.55228 10 7 10C6.44772 10 6 10.4477 6 11C6 11.5523 6.44772 12 7 12C7.55228 12 8 11.5523 8 11Z" fill="#52525E"/>
+<path d="M12 11C12 10.4477 11.5523 10 11 10C10.4477 10 10 10.4477 10 11C10 11.5523 10.4477 12 11 12C11.5523 12 12 11.5523 12 11Z" fill="#52525E"/>
+<path d="M51 10H15C14.4477 10 14 10.4477 14 11C14 11.5523 14.4477 12 15 12H51C51.5523 12 52 11.5523 52 11C52 10.4477 51.5523 10 51 10Z" fill="#52525E"/>
+<path d="M58 14H4V45H58V14Z" fill="#CFCFD8"/>
+<path d="M8 14H55V39C55 40.1046 54.1046 41 53 41H10C8.89543 41 8 40.1046 8 39V14Z" fill="white"/>
+<path d="M38 18H13C12.4477 18 12 18.4477 12 19C12 19.5523 12.4477 20 13 20H38C38.5523 20 39 19.5523 39 19C39 18.4477 38.5523 18 38 18Z" fill="#52525E"/>
+<path d="M27 22H13C12.4477 22 12 22.4477 12 23C12 23.5523 12.4477 24 13 24H27C27.5523 24 28 23.5523 28 23C28 22.4477 27.5523 22 27 22Z" fill="#52525E"/>
+<path d="M42 26H13C12.4477 26 12 26.4477 12 27C12 27.5523 12.4477 28 13 28H42C42.5523 28 43 27.5523 43 27C43 26.4477 42.5523 26 42 26Z" fill="#52525E"/>
+</g>
+<rect x="4.5" y="3.5" width="53" height="41" rx="3.5" stroke="#8F8F9D" shape-rendering="crispEdges"/>
+</g>
+<defs>
+<filter id="filter0_d_202_926" x="0" y="0" width="62" height="50" filterUnits="userSpaceOnUse" color-interpolation-filters="sRGB">
+<feFlood flood-opacity="0" result="BackgroundImageFix"/>
+<feColorMatrix in="SourceAlpha" type="matrix" values="0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 127 0" result="hardAlpha"/>
+<feOffset dy="1"/>
+<feGaussianBlur stdDeviation="2"/>
+<feComposite in2="hardAlpha" operator="out"/>
+<feColorMatrix type="matrix" values="0 0 0 0 0.0470588 0 0 0 0 0.0470588 0 0 0 0 0.0509804 0 0 0 0.1 0"/>
+<feBlend mode="normal" in2="BackgroundImageFix" result="effect1_dropShadow_202_926"/>
+<feBlend mode="normal" in="SourceGraphic" in2="effect1_dropShadow_202_926" result="shape"/>
+</filter>
+<clipPath id="clip0_202_926">
+<rect x="4" y="3" width="54" height="42" rx="4" fill="white"/>
+</clipPath>
+</defs>
+</svg>

browser/components/preferences/letterboxing.css

+.letterboxing-overview {
+  margin-block-end: 32px;
+}
+
+.letterboxing-search-overview {
+  margin-block-end: 16px;
+}
+#letterboxingAligner {
+  display: flex;
+  justify-content: start;
+  gap: 16px;
+  margin-block: 12px;
+
+  --letterboxing-alignment-img-top: url("chrome://browser/content/preferences/letterboxing-top-light.svg");
+  --letterboxing-alignment-img-middle: url("chrome://browser/content/preferences/letterboxing-middle-light.svg");
+}
+
+@media (prefers-color-scheme: dark) {
+  #letterboxingAligner  {
+    --letterboxing-alignment-img-top: url("chrome://browser/content/preferences/letterboxing-top-dark.svg");
+    --letterboxing-alignment-img-middle: url("chrome://browser/content/preferences/letterboxing-middle-dark.svg");
+  }
+}
+
+#letterboxingAligner .letterboxing-align-top img {
+  content: var(--letterboxing-alignment-img-top);
+}
+#letterboxingAligner .letterboxing-align-middle img {
+  content: var(--letterboxing-alignment-img-middle);
+}
+
+#letterboxingAligner label {
+  width: 200px;
+  flex: 0 0 auto;
+}
+
+#letterboxingAligner img {
+  border: none;
+  box-shadow: none;
+}
+
+/* Hide letterboxing options depending on whether ResistFingerprinting or
+ * Letterboxing is enabled. */
+#letterboxingCategory:not(.rfp-enabled),
+#letterboxingCategory:not(.rfp-enabled) ~ #letterboxingDisabled,
+#letterboxingCategory.letterboxing-enabled ~ #letterboxingDisabled,
+#letterboxingCategory:not(.rfp-enabled.letterboxing-enabled) ~ .letterboxing-category {
+  display: none;
+}
+
+/* We duplicate the Letterboxing overview to appear in search results.
+ * We want to hide them whenever they are *not* part of a search result. */
+:is(
+  /* Hide the duplicates when the category is *not* hidden. */
+  #letterboxingCategory:not(.visually-hidden) ~ .letterboxing-category,
+  /* Also hide the duplicated overview for subsequence search results if a
+   * previous groupbox is *not* hidden. */
+  .letterboxing-category:not(.visually-hidden) ~ .letterboxing-category,
+) .letterboxing-search-overview {
+  display: none;
+}
+
+/* Similarly for #letterboxingDisabled, but we also want to hide its
+ * "Letterboxing" heading.
+ * NOTE: This should only appear in isolation, so we do not need to worry about
+ * subsequent groupboxes. */
+#letterboxingCategory:not(.visually-hidden) ~ #letterboxingDisabled :is(
+  .letterboxing-search-heading,
+  .letterboxing-search-overview
+) {
+  display: none;
+}

browser/components/preferences/letterboxing.inc.xhtml

+<script src="chrome://browser/content/preferences/letterboxing.js" />
+<vbox
+  id="letterboxingCategory"
+  class="subcategory"
+  hidden="true"
+  data-category="paneGeneral"
+>
+  <html:h1 data-l10n-id="letterboxing-header" />
+  <description class="letterboxing-overview">
+    <html:span
+      data-l10n-id="letterboxing-overview"
+      class="tail-with-learn-more"
+    ></html:span>
+    <label
+      data-l10n-id="letterboxing-learn-more"
+      class="learnMore text-link"
+      is="text-link"
+      href="about:manual#letterboxing"
+      useoriginprincipal="true"
+    />
+  </description>
+</vbox>
+<groupbox
+  class="letterboxing-category"
+  data-category="paneGeneral"
+  hidden="true"
+>
+  <label><html:h2
+    data-l10n-id="letterboxing-window-size-header"/></label>
+  <description class="letterboxing-search-overview">
+    <html:span
+      data-l10n-id="letterboxing-overview"
+      class="tail-with-learn-more"
+    ></html:span>
+    <label
+      data-l10n-id="letterboxing-learn-more"
+      class="learnMore text-link"
+      is="text-link"
+      href="about:manual#letterboxing"
+      useoriginprincipal="true"
+    />
+  </description>
+  <checkbox
+    id="letterboxingRememberSize"
+    data-l10n-id="letterboxing-remember-size"
+    preference="privacy.resistFingerprinting.letterboxing.rememberSize"
+  />
+</groupbox>
+<groupbox
+  class="letterboxing-category"
+  data-category="paneGeneral"
+  hidden="true"
+>
+  <label><html:h2
+    data-l10n-id="letterboxing-alignment-header"
+    /></label>
+  <description class="letterboxing-search-overview">
+    <html:span
+      data-l10n-id="letterboxing-overview"
+      class="tail-with-learn-more"
+    ></html:span>
+    <label
+      data-l10n-id="letterboxing-learn-more"
+      class="learnMore text-link"
+      is="text-link"
+      href="about:manual#letterboxing"
+      useoriginprincipal="true"
+    />
+  </description>
+  <description
+    id="letterboxingAlignmentDesc"
+    flex="1"
+    data-l10n-id="letterboxing-alignment-description"
+  />
+  <form
+    xmlns="http://www.w3.org/1999/xhtml"
+    id="letterboxingAligner"
+    autocomplete="off"
+    aria-labelledby="letterboxingAlignmentDesc"
+  >
+    <label class="web-appearance-choice letterboxing-align-top">
+      <div class="web-appearance-choice-image-container">
+        <img role="presentation" />
+      </div>
+      <div class="web-appearance-choice-footer">
+        <input type="radio" name="alignment" value="top" />
+        <span data-l10n-id="letterboxing-alignment-top"></span>
+      </div>
+    </label>
+    <label class="web-appearance-choice letterboxing-align-middle">
+      <div class="web-appearance-choice-image-container">
+        <img role="presentation" />
+      </div>
+      <div class="web-appearance-choice-footer">
+        <input type="radio" name="alignment" value="middle" />
+        <span data-l10n-id="letterboxing-alignment-middle"></span>
+      </div>
+    </label>
+  </form>
+</groupbox>
+<groupbox
+  id="letterboxingDisabled"
+  data-category="paneGeneral"
+  hidden="true"
+>
+  <label class="letterboxing-search-heading"><html:h2
+    data-l10n-id="letterboxing-header"/></label>
+  <description class="letterboxing-search-overview">
+    <html:span
+      data-l10n-id="letterboxing-overview"
+      class="tail-with-learn-more"
+    ></html:span>
+    <label
+      data-l10n-id="letterboxing-learn-more"
+      class="learnMore text-link"
+      is="text-link"
+      href="about:manual#letterboxing"
+      useoriginprincipal="true"
+    />
+  </description>
+  <hbox align="center">
+    <label
+      class="face-sad"
+      flex="1"
+      data-l10n-id="letterboxing-disabled-description"
+    />
+    <button
+      id="letterboxingEnableButton"
+      class="accessory-button"
+      is="highlightable-button"
+      data-l10n-id="letterboxing-enable-button"
+    />
+  </hbox>
+</groupbox>

browser/components/preferences/letterboxing.js

+/* import-globals-from /browser/components/preferences/preferences.js */
+/* import-globals-from /browser/components/preferences/findInPage.js */
+/* import-globals-from /toolkit/content/preferencesBindings.js */
+
+Preferences.addAll([
+  {
+    id: "privacy.resistFingerprinting.letterboxing.rememberSize",
+    type: "bool",
+  },
+]);
+
+{
+  const lbEnabledPref = "privacy.resistFingerprinting.letterboxing";
+  const visibilityPrefs = ["privacy.resistFingerprinting", lbEnabledPref];
+  const alignMiddlePref = "privacy.resistFingerprinting.letterboxing.vcenter";
+
+  const hideFromSearchIf = (mustHide, ...elements) => {
+    for (const element of elements) {
+      if (mustHide) {
+        element.setAttribute("data-hidden-from-search", "true");
+      } else {
+        element.removeAttribute("data-hidden-from-search");
+      }
+    }
+  };
+
+  const syncVisibility = () => {
+    const [rfpEnabled, letterboxingEnabled] = visibilityPrefs.map(pref =>
+      Services.prefs.getBoolPref(pref, false)
+    );
+    const categoryElement = document.getElementById("letterboxingCategory");
+    const { classList } = categoryElement;
+
+    // Show the letterboxing section only if resistFingerprinting is enabled
+    classList.toggle("rfp-enabled", rfpEnabled);
+    classList.toggle("letterboxing-enabled", letterboxingEnabled);
+
+    // To ensure the hidden parts do not contribute to search results, we need
+    // to add "data-hidden-from-search".
+    hideFromSearchIf(
+      !rfpEnabled || !letterboxingEnabled,
+      ...document.querySelectorAll(".letterboxing-category")
+    );
+    hideFromSearchIf(
+      !rfpEnabled || letterboxingEnabled,
+      document.getElementById("letterboxingDisabled")
+    );
+  };
+
+  const onVisibilityPrefChange = () => {
+    syncVisibility();
+    // NOTE: Firefox does not expect "data-hidden-from-search" to change
+    // dynamically after page initialization. So we need to manually recall the
+    // methods that use "data-hidden-from-search". I.e. the "search" method,
+    // using the currently shown category.
+    // NOTE: We skip this if we are just initializing on page load.
+    // NOTE: data-hidden-from-search is also used when the user has entered a
+    // search term. We do not update the results in this case. Instead, it will
+    // update when the search term changes or is cleared.
+    if (!gSearchResultsPane.query) {
+      search(gLastCategory.category, "data-category");
+    }
+  };
+
+  const alignerId = "letterboxingAligner";
+  const syncAligner = () => {
+    const value = Services.prefs.getBoolPref(alignMiddlePref)
+      ? "middle"
+      : "top";
+    document.querySelector(
+      `#${alignerId} input[value="${value}"]`
+    ).checked = true;
+  };
+
+  var gLetterboxingPrefs = {
+    init() {
+      syncVisibility();
+      document
+        .getElementById("letterboxingEnableButton")
+        .addEventListener("command", () => {
+          Services.prefs.setBoolPref(lbEnabledPref, true);
+          // Button should have focus when activated but will be hidden now,
+          // so re-assign focus to the newly revealed options.
+          Services.focus.moveFocus(
+            window,
+            document.querySelector(".letterboxing-category"),
+            Services.focus.MOVEFOCUS_FIRST,
+            0
+          );
+        });
+      for (const pref of visibilityPrefs) {
+        Services.prefs.addObserver(pref, onVisibilityPrefChange);
+      }
+
+      syncAligner();
+      document.getElementById(alignerId).addEventListener("change", e => {
+        // NOTE: the "change" event is only fired on the checked input.
+        Services.prefs.setBoolPref(
+          alignMiddlePref,
+          e.target.value === "middle"
+        );
+      });
+      Services.prefs.addObserver(alignMiddlePref, syncAligner);
+    },
+
+    destroy() {
+      for (const pref of visibilityPrefs) {
+        Services.prefs.removeObserver(pref, onVisibilityPrefChange);
+      }
+      Services.prefs.removeObserver(alignMiddlePref, syncAligner);
+    },
+  };
+}

browser/components/preferences/main.inc.xhtml

@@ -130,6 +130,9 @@
     </vbox>
 </groupbox>
 
+<!-- Letterboxing -->
+#include ./letterboxing.inc.xhtml
+
 <hbox id="languageAndAppearanceCategory"
       class="subcategory"
       hidden="true"

browser/components/preferences/main.js

@@ -729,6 +729,7 @@ var gMainPane = {
     ]);
 
     AppearanceChooser.init();
+    gLetterboxingPrefs.init();
 
     // Notify observers that the UI is now ready
     Services.obs.notifyObservers(window, "main-pane-loaded");
@@ -2585,6 +2586,7 @@ var gMainPane = {
     Services.prefs.removeObserver(PREF_CONTAINERS_EXTENSION, this);
     Services.obs.removeObserver(this, AUTO_UPDATE_CHANGED_TOPIC);
     Services.obs.removeObserver(this, BACKGROUND_UPDATE_CHANGED_TOPIC);
+    gLetterboxingPrefs.destroy();
     AppearanceChooser.destroy();
   },
 

browser/components/preferences/preferences.xhtml

@@ -12,6 +12,7 @@
 <?xml-stylesheet href="chrome://browser/skin/preferences/search.css"?>
 <?xml-stylesheet href="chrome://browser/skin/preferences/containers.css"?>
 <?xml-stylesheet href="chrome://browser/skin/preferences/privacy.css"?>
+<?xml-stylesheet href="chrome://browser/content/preferences/letterboxing.css"?>
 <?xml-stylesheet href="chrome://browser/content/securitylevel/securityLevelPreferences.css"?>
 <?xml-stylesheet href="chrome://browser/content/torpreferences/torPreferences.css"?>
 <?xml-stylesheet href="chrome://browser/content/onionservices/authPreferences.css"?>
@@ -42,6 +43,7 @@
 
   <!-- Links below are only used for search-l10n-ids into subdialogs -->
   <link rel="localization" href="browser/aboutDialog.ftl"/>
+  <link rel="localization" href="browser/base-browser.ftl"/>
   <link rel="localization" href="browser/preferences/addEngine.ftl"/>
   <link rel="localization" href="browser/preferences/blocklists.ftl"/>
   <link rel="localization" href="browser/preferences/clearSiteData.ftl"/>