Bug 1997455 - Add a pref so forced-colors theme override can optionally be disabled r=emilio,rpl

Differential Revision: https://phabricator.services.mozilla.com/D271323

Author: jaredhirsch

browser/app/profile/firefox.js

@@ -905,6 +905,10 @@ pref("browser.dataFeatureRecommendations.enabled", false);
 // sets darkTheme data.
 pref("browser.theme.dark-private-windows", true);
 
+// Whether to override themes in forced-colors mode and just use the
+// system theme and forced-colors palette to style the chrome.
+pref("browser.theme.forced-colors-override.enabled", true);
+
 // Pref to control whether or not Private Browsing windows show up
 // as separate icons in the Windows taskbar.
 pref("browser.privateWindowSeparation.enabled", true);

toolkit/modules/LightweightThemeConsumer.sys.mjs

@@ -222,6 +222,14 @@ export function LightweightThemeConsumer(aDocument) {
   this._win = aDocument.defaultView;
   this._winId = this._win.docShell.outerWindowID;
 
+  XPCOMUtils.defineLazyPreferenceGetter(
+    this,
+    "FORCED_COLORS_OVERRIDE_ENABLED",
+    "browser.theme.forced-colors-override.enabled",
+    true,
+    () => this._update(this._lastData)
+  );
+
   Services.obs.addObserver(this, "lightweight-theme-styling-update");
 
   this.darkThemeMediaQuery = this._win.matchMedia("(-moz-system-dark-theme)");
@@ -309,7 +317,10 @@ LightweightThemeConsumer.prototype = {
     })();
 
     let theme = useDarkTheme ? themeData.darkTheme : themeData.theme;
-    if (!theme || this.forcedColorsMediaQuery?.matches) {
+    let forcedColorsThemeOverride =
+      this.FORCED_COLORS_OVERRIDE_ENABLED &&
+      this.forcedColorsMediaQuery?.matches;
+    if (!theme || forcedColorsThemeOverride) {
       theme = { id: DEFAULT_THEME_ID };
     }
     let builtinThemeConfig = lazy.BuiltInThemeConfig.get(theme.id);

toolkit/mozapps/extensions/content/aboutaddons.js

@@ -57,6 +57,12 @@ XPCOMUtils.defineLazyPreferenceGetter(
   "extensions.dataCollectionPermissions.enabled",
   false
 );
+XPCOMUtils.defineLazyPreferenceGetter(
+  this,
+  "FORCED_COLORS_OVERRIDE_ENABLED",
+  "browser.theme.forced-colors-override.enabled",
+  true
+);
 
 const PLUGIN_ICON_URL = "chrome://global/skin/icons/plugin.svg";
 const EXTENSION_ICON_URL =
@@ -4172,8 +4178,10 @@ class ForcedColorsNotice extends HTMLElement {
   }
 
   render() {
-    this.hidden = !this.forcedColorsMediaQuery.matches;
-    if (!this.hidden && this.childElementCount == 0) {
+    let shouldShowNotice =
+      FORCED_COLORS_OVERRIDE_ENABLED && this.forcedColorsMediaQuery.matches;
+    this.hidden = !shouldShowNotice;
+    if (shouldShowNotice && this.childElementCount == 0) {
       this.appendChild(importTemplate("forced-colors-notice"));
     }
   }

toolkit/mozapps/extensions/test/browser/browser_html_forced_colors_notice.js

@@ -63,5 +63,17 @@ add_task(async function test_aboutaddons_forced_colors_notice() {
   });
   await assertForcedColorsNotice(win, { expectVisible: false });
 
+  info("Test that forced-colors override is disabled when the pref is false");
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["browser.theme.forced-colors-override.enabled", false],
+      ["ui.useAccessibilityTheme", 1],
+    ],
+  });
+  await closeView(win);
+  win = await loadInitialView("theme");
+  await assertForcedColorsNotice(win, { expectVisible: false });
+  await SpecialPowers.popPrefEnv();
+
   await closeView(win);
 });

toolkit/mozapps/extensions/test/browser/browser_theme_forced_colors.js

@@ -46,5 +46,18 @@ add_task(async function test_forced_colors_themes() {
     "should be in forced-colors mode"
   );
 
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.theme.forced-colors-override.enabled", false]],
+  });
+
+  Assert.ok(
+    docEl.hasAttribute("lwtheme"),
+    "when forced-colors override is disabled, LWT attribute should be set"
+  );
+  Assert.ok(
+    window.matchMedia("(forced-colors)").matches,
+    "when forced-colors override is disabled, window should still be in forced-colors mode"
+  );
+
   await theme.unload();
 });