Add QPlatformTheme::MouseCursorTheme and MouseCursorSize

Also implement in QGenericUnixTheme/QKdeTheme/QGnomeTheme/QGtk3Theme.

On Wayland, there is wl_cursor_theme_load() and etc in wayland-cursor.h.

On X11/xcb, cursor theme and size are common, either in XCURSOR_THEME
and XCURSOR_SIZE envs, or in gsettings for gtk/gnome world.

On Windows, cursor theme is part of theme file, see SetWindowTheme in
https://learn.microsoft.com/en-gb/windows/win32/api/uxtheme/nf-uxtheme-setwindowtheme .
For system settings, for example on Windows 10, Mouse Settings,
Additional mouse options, Pointers Tab, Scheme.

On macOS, it looks like cursor theme is deprecated.

Change-Id: I5821377d966c281fb8330da1f5baa7f0ddf86a9f
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qplatformtheme.cpp

@@ -147,6 +147,12 @@ QT_BEGIN_NAMESPACE
            input focus after a touch/mouse release.
            This enum value has been added in Qt 6.5.
 
+    \value MouseCursorTheme (QString) Name of the mouse cursor theme.
+           This enum value has been added in Qt 6.5.
+
+    \value MouseCursorSize (QSize) Size of the mouse cursor.
+           This enum value has been added in Qt 6.5.
+
     \sa themeHint(), QStyle::pixelMetric()
 */
 
@@ -627,6 +633,10 @@ QVariant QPlatformTheme::defaultThemeHint(ThemeHint hint)
         return QVariant(5000);
     case MenuBarFocusOnAltPressRelease:
         return false;
+    case MouseCursorTheme:
+        return QVariant(QString());
+    case MouseCursorSize:
+        return QVariant(QSize(16, 16));
     }
     return QVariant();
 }

src/gui/kernel/qplatformtheme.h

@@ -92,7 +92,9 @@ public:
         FlickStartDistance,
         FlickMaximumVelocity,
         FlickDeceleration,
-        MenuBarFocusOnAltPressRelease
+        MenuBarFocusOnAltPressRelease,
+        MouseCursorTheme,
+        MouseCursorSize
     };
     Q_ENUM(ThemeHint)
 

src/gui/platform/unix/qgenericunixthemes.cpp

@@ -86,6 +86,20 @@ static bool isDBusTrayAvailable() {
 }
 #endif
 
+static QString mouseCursorTheme()
+{
+    static QString themeName = qEnvironmentVariable("XCURSOR_THEME");
+    return themeName;
+}
+
+static QSize mouseCursorSize()
+{
+    constexpr int defaultCursorSize = 24;
+    static const int xCursorSize = qEnvironmentVariableIntValue("XCURSOR_SIZE");
+    static const int s = xCursorSize > 0 ? xCursorSize : defaultCursorSize;
+    return QSize(s, s);
+}
+
 #ifndef QT_NO_DBUS
 static bool checkDBusGlobalMenuAvailable()
 {
@@ -290,6 +304,10 @@ QVariant QGenericUnixTheme::themeHint(ThemeHint hint) const
         return QVariant(int(X11KeyboardScheme));
     case QPlatformTheme::UiEffects:
         return QVariant(int(HoverEffect));
+    case QPlatformTheme::MouseCursorTheme:
+        return QVariant(mouseCursorTheme());
+    case QPlatformTheme::MouseCursorSize:
+        return QVariant(mouseCursorSize());
     default:
         break;
     }
@@ -711,6 +729,10 @@ QVariant QKdeTheme::themeHint(QPlatformTheme::ThemeHint hint) const
         return QVariant(d->cursorBlinkRate);
     case QPlatformTheme::UiEffects:
         return QVariant(int(HoverEffect));
+    case QPlatformTheme::MouseCursorTheme:
+        return QVariant(mouseCursorTheme());
+    case QPlatformTheme::MouseCursorSize:
+        return QVariant(mouseCursorSize());
     default:
         break;
     }
@@ -983,6 +1005,10 @@ QVariant QGnomeTheme::themeHint(QPlatformTheme::ThemeHint hint) const
                 QList<Qt::Key>({ Qt::Key_Space, Qt::Key_Return, Qt::Key_Enter, Qt::Key_Select }));
     case QPlatformTheme::PreselectFirstFileInDirectory:
         return true;
+    case QPlatformTheme::MouseCursorTheme:
+        return QVariant(mouseCursorTheme());
+    case QPlatformTheme::MouseCursorSize:
+        return QVariant(mouseCursorSize());
     default:
         break;
     }

src/plugins/platformthemes/gtk3/qgtk3theme.cpp

@@ -101,6 +101,8 @@ QGtk3Theme::QGtk3Theme()
     SETTING_CONNECT("gtk-font-name");
     SETTING_CONNECT("gtk-application-prefer-dark-theme");
     SETTING_CONNECT("gtk-theme-name");
+    SETTING_CONNECT("gtk-cursor-theme-name");
+    SETTING_CONNECT("gtk-cursor-theme-size");
 #undef SETTING_CONNECT
 
     /* Set XCURSOR_SIZE and XCURSOR_THEME for Wayland sessions */
@@ -150,6 +152,14 @@ QVariant QGtk3Theme::themeHint(QPlatformTheme::ThemeHint hint) const
         return QVariant(gtkSetting("gtk-icon-theme-name"));
     case QPlatformTheme::SystemIconFallbackThemeName:
         return QVariant(gtkSetting("gtk-fallback-icon-theme"));
+    case QPlatformTheme::MouseCursorTheme:
+        return QVariant(gtkSetting("gtk-cursor-theme-name"));
+    case QPlatformTheme::MouseCursorSize: {
+        int s = gtkSetting<gint>("gtk-cursor-theme-size");
+        if (s > 0)
+            return QVariant(QSize(s, s));
+        return QGnomeTheme::themeHint(hint);
+    }
     default:
         return QGnomeTheme::themeHint(hint);
     }