Add QPlatformTheme::MenuBarFocusOnAltPressRelease

On Windows, pressing and releasing the Alt key moves focus to the
menubar. This is implemented in widgets, but not in Qt Quick.

Add a new theme hint so that we can make Qt behave natively on all
platforms, and make the common style's respective styleHint default to
the theme hint.

Change-Id: I101bec56afd51e81ebb128c288f8a1e9b7efedb4
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qplatformtheme.cpp

@@ -625,6 +625,8 @@ QVariant QPlatformTheme::defaultThemeHint(ThemeHint hint)
         return QVariant(2500);
     case FlickDeceleration:
         return QVariant(5000);
+    case MenuBarFocusOnAltPressRelease:
+        return false;
     }
     return QVariant();
 }

src/gui/kernel/qplatformtheme.h

@@ -91,7 +91,8 @@ public:
         SetFocusOnTouchRelease,
         FlickStartDistance,
         FlickMaximumVelocity,
-        FlickDeceleration
+        FlickDeceleration,
+        MenuBarFocusOnAltPressRelease
     };
     Q_ENUM(ThemeHint)
 

src/plugins/platforms/windows/qwindowstheme.cpp

@@ -558,6 +558,8 @@ QVariant QWindowsTheme::themeHint(ThemeHint hint) const
     }
     case MouseDoubleClickDistance:
         return GetSystemMetrics(SM_CXDOUBLECLK);
+    case MenuBarFocusOnAltPressRelease:
+        return true;
     default:
         break;
     }

src/widgets/styles/qcommonstyle.cpp

@@ -5384,7 +5384,6 @@ int QCommonStyle::styleHint(StyleHint sh, const QStyleOption *opt, const QWidget
     case SH_Menu_AllowActiveAndDisabled:
     case SH_Menu_SpaceActivatesItem:
     case SH_ScrollView_FrameOnlyAroundContents:
-    case SH_MenuBar_AltKeyNavigation:
     case SH_ComboBox_ListMouseTracking:
     case SH_Menu_MouseTracking:
     case SH_MenuBar_MouseTracking:
@@ -5410,6 +5409,10 @@ int QCommonStyle::styleHint(StyleHint sh, const QStyleOption *opt, const QWidget
     case SH_Table_AlwaysDrawLeftTopGridLines:
         ret = false;
         break;
+    case SH_MenuBar_AltKeyNavigation:
+        if (const QPlatformTheme *theme = QGuiApplicationPrivate::platformTheme())
+            ret = theme->themeHint(QPlatformTheme::MenuBarFocusOnAltPressRelease).toBool();
+        break;
     case SH_CustomBase:
         // Added to get compiler errors when a style hint is missing
         ret = false;