[mutter/gnome-3-26] backends/native: Fix handling of trackball settings

[Carlos Garnacho <carlosg src gnome org>]

commit 6ef08c5018c8311653b49f85d3545facf0517279
Author: Carlos Garnacho <carlosg gnome org>
Date:   Wed Oct 4 18:58:13 2017 +0200

    backends/native: Fix handling of trackball settings
    
    The org.gnome.desktop.peripherals.trackball.scroll-wheel-emulation-button
    setting contains buttons X11-style. Work out the BTN evcode that applies
    to it when applying the setting on the libinput device.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=787804

 src/backends/native/meta-input-settings-native.c |   23 +++++++++++++++++++--
 1 files changed, 20 insertions(+), 3 deletions(-)
---
diff --git a/src/backends/native/meta-input-settings-native.c 
b/src/backends/native/meta-input-settings-native.c
index 8a62bb3..beb7217 100644
--- a/src/backends/native/meta-input-settings-native.c
+++ b/src/backends/native/meta-input-settings-native.c
@@ -245,16 +245,33 @@ meta_input_settings_native_set_scroll_button (MetaInputSettings  *settings,
                                               guint               button)
 {
   struct libinput_device *libinput_device;
+  enum libinput_config_scroll_method method;
+  guint evcode;
 
   libinput_device = clutter_evdev_input_device_get_libinput_device (device);
   if (!libinput_device)
     return;
 
-  if (!device_set_scroll_method (libinput_device,
-                                 LIBINPUT_CONFIG_SCROLL_ON_BUTTON_DOWN))
+  if (button == 0)
+    {
+      method = LIBINPUT_CONFIG_SCROLL_NO_SCROLL;
+      evcode = 0;
+    }
+  else
+    {
+      /* Compensate for X11 scroll buttons */
+      if (button > 7)
+        button -= 4;
+
+      /* Button is 1-indexed */
+      evcode = (BTN_LEFT - 1) + button;
+      method = LIBINPUT_CONFIG_SCROLL_ON_BUTTON_DOWN;
+    }
+
+  if (!device_set_scroll_method (libinput_device, method))
     return;
 
-  libinput_device_config_scroll_set_button (libinput_device, button);
+  libinput_device_config_scroll_set_button (libinput_device, evcode);
 }
 
 static void