helium/ui: add MRU tab cycling popup behind experimental flag

Add a centered popup overlay for MRU tab cycling, gated behind
the --helium-tab-cycling-popup chrome://flags entry (disabled by
default).

When the flag is enabled and Ctrl+Tab is pressed with MRU cycling
active, a popup lists tabs in most-recently-used order. The highlight
moves with each Tab press and activation is deferred until Ctrl is
released. When the flag is disabled, tab cycling behaves as before
with immediate activation on each step.

Author: markwai

i18n/source.gen.json

@@ -557,6 +557,24 @@
     "context": "Description for the zen mode toggle in appearance settings.",
     "message": "Hide toolbar and tab strip until hovering near their edges"
   },
+  {
+    "name": "IDS_TAB_CYCLING_POPUP_ACCESSIBLE_NAME",
+    "source": "chrome/app/settings_strings.grdp",
+    "context": "Screen reader label for the MRU tab cycling popup.",
+    "message": "Recently used tabs"
+  },
+  {
+    "name": "IDS_TAB_CYCLING_AND_MORE_TABS",
+    "source": "chrome/app/settings_strings.grdp",
+    "context": "Footer shown at the bottom of the MRU tab cycling popup when additional tabs are not visible.",
+    "message": "{COUNT, plural,=1 {and 1 more...}other {and # more...}}"
+  },
+  {
+    "name": "IDS_TAB_CYCLING_UNTITLED_TAB",
+    "source": "chrome/app/settings_strings.grdp",
+    "context": "Fallback label in the MRU tab cycling popup when a tab has no title.",
+    "message": "Untitled"
+  },
   {
     "name": "IDS_SETTINGS_DEFAULT_BROWSER_SECONDARY",
     "source": "chrome/app/settings_chromium_strings.grdp",

patches/helium/ui/experiments/tab-cycling-popup-wiring.patch

@@ -0,0 +1,50 @@
+--- a/chrome/browser/ui/ui_features.cc
++++ b/chrome/browser/ui/ui_features.cc
+@@ -32,6 +32,12 @@ bool IsHeliumZenModeFeatureEnabled() {
+   return base::FeatureList::IsEnabled(kHeliumZenMode);
+ }
+ 
++BASE_FEATURE(kHeliumTabCyclingPopup, base::FEATURE_DISABLED_BY_DEFAULT);
++
++bool IsHeliumTabCyclingPopupFeatureEnabled() {
++  return base::FeatureList::IsEnabled(kHeliumTabCyclingPopup);
++}
++
+ // Enables the use of WGC for the Eye Dropper screen capture.
+ BASE_FEATURE(kAllowEyeDropperWGCScreenCapture,
+ #if BUILDFLAG(IS_WIN)
+--- a/chrome/browser/ui/ui_features.h
++++ b/chrome/browser/ui/ui_features.h
+@@ -25,6 +25,9 @@ bool HeliumUseCompactLocationWidth();
+ BASE_DECLARE_FEATURE(kHeliumZenMode);
+ bool IsHeliumZenModeFeatureEnabled();
+ 
++BASE_DECLARE_FEATURE(kHeliumTabCyclingPopup);
++bool IsHeliumTabCyclingPopupFeatureEnabled();
++
+ BASE_DECLARE_FEATURE(kAllowEyeDropperWGCScreenCapture);
+ 
+ BASE_DECLARE_FEATURE(kBrowserWidgetCacheThemeService);
+--- a/chrome/browser/helium_flag_choices.h
++++ b/chrome/browser/helium_flag_choices.h
+@@ -38,6 +38,8 @@ namespace helium {
+   constexpr const char kHeliumCompactLocationWidthCommandLine[] =
+       "helium-compact-location-width";
+   constexpr const char kHeliumZenModeCommandLine[] = "helium-zen-mode";
++  constexpr const char kHeliumTabCyclingPopupCommandLine[] =
++      "helium-tab-cycling-popup";
+ }  // namespace helium
+ 
+ #endif  /* CHROME_BROWSER_HELIUM_FLAG_CHOICES_H_ */
+--- a/chrome/browser/helium_flag_entries.h
++++ b/chrome/browser/helium_flag_entries.h
+@@ -46,4 +46,9 @@
+      "Hides browser chrome (toolbar, tab strip, bookmarks bar) and reveals it "
+      "on hover near the window edges. Helium flag.",
+      kOsDesktop, FEATURE_VALUE_TYPE(features::kHeliumZenMode)},
++    {helium::kHeliumTabCyclingPopupCommandLine,
++     "Tab Cycling Popup",
++     "Shows a popup to select which tab to switch to when cycling tabs "
++     "in most recently used order. Helium flag.",
++     kOsDesktop, FEATURE_VALUE_TYPE(features::kHeliumTabCyclingPopup)},
+ #endif  /* CHROME_BROWSER_HELIUM_FLAG_ENTRIES_H_ */