backends: add support for scroll button locking

Where enabled, the first click of the scroll button sets the button logically
down, the second click sets the button logically up.

This is an accessibility feature, it doesn't require users to keep holding the
button down while scrolling which is hard or impossible for some users.

gsettings-desktop-schemas merge request:
https://gitlab.gnome.org/GNOME/gsettings-desktop-schemas/-/merge_requests/39

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1432

meson.build

@@ -44,7 +44,7 @@ wayland_server_req = '>= 1.18'
 wayland_protocols_req = '>= 1.19'
 
 # native backend version requirements
-libinput_req = '>= 1.7'
+libinput_req = '>= 1.15.0'
 gbm_req = '>= 17.3'
 
 # screen cast version requirements

src/backends/meta-input-settings-private.h

@@ -78,7 +78,8 @@ struct _MetaInputSettingsClass
                                   gboolean            enabled);
   void (* set_scroll_button) (MetaInputSettings  *settings,
                               ClutterInputDevice *device,
-                              guint               button);
+                              guint               button,
+                              gboolean            button_lock);
 
   void (* set_click_method)  (MetaInputSettings            *settings,
                               ClutterInputDevice           *device,

src/backends/meta-input-settings.c

@@ -858,6 +858,7 @@ update_trackball_scroll_button (MetaInputSettings  *input_settings,
   MetaInputSettingsClass *input_settings_class;
   MetaInputSettingsPrivate *priv;
   guint button;
+  gboolean button_lock;
 
   priv = meta_input_settings_get_instance_private (input_settings);
   input_settings_class = META_INPUT_SETTINGS_GET_CLASS (input_settings);
@@ -868,10 +869,11 @@ update_trackball_scroll_button (MetaInputSettings  *input_settings,
   /* This key is 'i' in the schema but it also specifies a minimum
    * range of 0 so the cast here is safe. */
   button = (guint) g_settings_get_int (priv->trackball_settings, "scroll-wheel-emulation-button");
+  button_lock = g_settings_get_boolean (priv->trackball_settings, "scroll-wheel-emulation-button-lock");
 
   if (device)
     {
-      input_settings_class->set_scroll_button (input_settings, device, button);
+      input_settings_class->set_scroll_button (input_settings, device, button, button_lock);
     }
   else if (!device)
     {
@@ -884,7 +886,7 @@ update_trackball_scroll_button (MetaInputSettings  *input_settings,
           device = l->data;
 
           if (input_settings_class->is_trackball_device (input_settings, device))
-            input_settings_class->set_scroll_button (input_settings, device, button);
+            input_settings_class->set_scroll_button (input_settings, device, button, button_lock);
         }
 
       g_list_free (devices);
@@ -1282,7 +1284,8 @@ meta_input_settings_changed_cb (GSettings  *settings,
     }
   else if (settings == priv->trackball_settings)
     {
-      if (strcmp (key, "scroll-wheel-emulation-button") == 0)
+      if (strcmp (key, "scroll-wheel-emulation-button") == 0 ||
+          strcmp (key, "scroll-wheel-emulation-button-lock") == 0)
         update_trackball_scroll_button (input_settings, NULL);
       else if (strcmp (key, "accel-profile") == 0)
         update_pointer_accel_profile (input_settings, settings, NULL);

src/backends/native/meta-input-settings-native.c

@@ -270,10 +270,12 @@ meta_input_settings_native_has_two_finger_scroll (MetaInputSettings  *settings,
 static void
 meta_input_settings_native_set_scroll_button (MetaInputSettings  *settings,
                                               ClutterInputDevice *device,
-                                              guint               button)
+                                              guint               button,
+                                              gboolean            button_lock)
 {
   struct libinput_device *libinput_device;
   enum libinput_config_scroll_method method;
+  enum libinput_config_scroll_button_lock_state lock_state;
   guint evcode;
 
   libinput_device = meta_input_device_native_get_libinput_device (device);
@@ -314,6 +316,13 @@ meta_input_settings_native_set_scroll_button (MetaInputSettings  *settings,
     return;
 
   libinput_device_config_scroll_set_button (libinput_device, evcode);
+
+  if (button_lock)
+    lock_state = LIBINPUT_CONFIG_SCROLL_BUTTON_LOCK_ENABLED;
+  else
+    lock_state = LIBINPUT_CONFIG_SCROLL_BUTTON_LOCK_DISABLED;
+
+  libinput_device_config_scroll_set_button_lock (libinput_device, lock_state);
 }
 
 static void

src/backends/x11/meta-input-settings-x11.c

@@ -375,11 +375,16 @@ meta_input_settings_x11_has_two_finger_scroll (MetaInputSettings  *settings,
 static void
 meta_input_settings_x11_set_scroll_button (MetaInputSettings  *settings,
                                            ClutterInputDevice *device,
-                                           guint               button)
+                                           guint               button,
+                                           gboolean            button_lock)
 {
+  gchar lock = button_lock;
+
   change_scroll_method (device, SCROLL_METHOD_FIELD_BUTTON, button != 0);
   change_property (device, "libinput Button Scrolling Button",
                    XA_CARDINAL, 32, &button, 1);
+  change_property (device, "libinput Button Scrolling Button Lock Enabled",
+                   XA_INTEGER, 8, &lock, 1);
 }
 
 static void