remote-desktop: Allow using custom scroll source for NotifyPointerAxis

Currently, the NotifyPointerAxis method always assumes that the scroll
source is CLUTTER_SCROLL_SOURCE_FINGER.
This is however not always true and in some cases a remote desktop
client needs to submit a PointerAxis event with a custom axis step.
This is for example the case with high resolution mouse wheels, where
the NotifyPointerAxisDiscrete method is unsuitable.
In such cases NotifyPointerAxis needs to be called, but with the
intention that the scroll source is still a mouse wheel.

To solve this situation, don't assume the scroll source always to be
CLUTTER_SCROLL_SOURCE_FINGER.
Instead, add further flag options to NotifyPointerAxis, which allow a
remote desktop client to choose the scroll source.
This way a remote desktop client can choose what scroll source is the
most suitable one for the current scroll event.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1636>

src/backends/meta-remote-desktop-session.c

@@ -39,9 +39,13 @@
 
 #define META_REMOTE_DESKTOP_SESSION_DBUS_PATH "/org/gnome/Mutter/RemoteDesktop/Session"
 
-enum _MetaRemoteDesktopNotifyAxisFlags
+typedef enum _MetaRemoteDesktopNotifyAxisFlags
 {
+  META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_NONE = 0,
   META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_FINISH = 1 << 0,
+  META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_WHEEL = 1 << 1,
+  META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_FINGER = 1 << 2,
+  META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_CONTINUOUS = 1 << 3,
 } MetaRemoteDesktopNotifyAxisFlags;
 
 struct _MetaRemoteDesktopSession
@@ -452,6 +456,33 @@ handle_notify_pointer_button (MetaDBusRemoteDesktopSession *skeleton,
   return TRUE;
 }
 
+static gboolean
+clutter_scroll_source_from_axis_flags (MetaRemoteDesktopNotifyAxisFlags  axis_flags,
+                                       ClutterScrollSource              *scroll_source)
+{
+  MetaRemoteDesktopNotifyAxisFlags scroll_mask;
+
+  scroll_mask = META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_WHEEL |
+                META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_FINGER |
+                META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_CONTINUOUS;
+
+  switch (axis_flags & scroll_mask)
+    {
+    case META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_WHEEL:
+      *scroll_source = CLUTTER_SCROLL_SOURCE_WHEEL;
+      return TRUE;
+    case META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_NONE:
+    case META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_FINGER:
+      *scroll_source = CLUTTER_SCROLL_SOURCE_FINGER;
+      return TRUE;
+    case META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_SOURCE_CONTINUOUS:
+      *scroll_source = CLUTTER_SCROLL_SOURCE_CONTINUOUS;
+      return TRUE;
+    }
+
+  return FALSE;
+}
+
 static gboolean
 handle_notify_pointer_axis (MetaDBusRemoteDesktopSession *skeleton,
                             GDBusMethodInvocation        *invocation,
@@ -461,10 +492,19 @@ handle_notify_pointer_axis (MetaDBusRemoteDesktopSession *skeleton,
 {
   MetaRemoteDesktopSession *session = META_REMOTE_DESKTOP_SESSION (skeleton);
   ClutterScrollFinishFlags finish_flags = CLUTTER_SCROLL_FINISHED_NONE;
+  ClutterScrollSource scroll_source;
 
   if (!meta_remote_desktop_session_check_can_notify (session, invocation))
     return TRUE;
 
+  if (!clutter_scroll_source_from_axis_flags (flags, &scroll_source))
+    {
+      g_dbus_method_invocation_return_error (invocation, G_DBUS_ERROR,
+                                             G_DBUS_ERROR_FAILED,
+                                             "Invalid scroll source");
+      return TRUE;
+    }
+
   if (flags & META_REMOTE_DESKTOP_NOTIFY_AXIS_FLAGS_FINISH)
     {
       finish_flags |= (CLUTTER_SCROLL_FINISHED_HORIZONTAL |
@@ -474,7 +514,7 @@ handle_notify_pointer_axis (MetaDBusRemoteDesktopSession *skeleton,
   clutter_virtual_input_device_notify_scroll_continuous (session->virtual_pointer,
                                                          CLUTTER_CURRENT_TIME,
                                                          dx, dy,
-                                                         CLUTTER_SCROLL_SOURCE_FINGER,
+                                                         scroll_source,
                                                          finish_flags);
 
   meta_dbus_remote_desktop_session_complete_notify_pointer_axis (skeleton,

src/org.gnome.Mutter.RemoteDesktop.xml

@@ -115,6 +115,15 @@
 
 	Possible @flags:
 	  1: finish - scroll motion was finished (e.g. fingers lifted)
+	  2: source_wheel - The scroll event is originated by a mouse wheel.
+	  4: source_finger - The scroll event is originated by one or more fingers on
+	                     the device (eg. touchpads).
+	  8: source_continuous - The scroll event is originated by the motion of some
+	                          device (eg. a scroll button is set).
+
+	  Maximum one of the @flags 'source_wheel', 'source_finger',
+	  'source_continuous' may be specified.
+	  If no source flag is specified, `source_finger` is assumed.
      -->
     <method name="NotifyPointerAxis">
       <arg name="dx" type="d" direction="in" />