Bug 1617803 - Support 4th and 5th mouse button disabling pref on Linux r=karlt

Some users may want to disable 4th and 5th button of mouse.
This feature has already been implemented on Windows, and
our GTK widget can support this with this simple patch.

Differential Revision: https://phabricator.services.mozilla.com/D64721

--HG--
extra : moz-landing-system : lando

modules/libpref/init/all.js

@@ -2387,7 +2387,7 @@ pref("middlemouse.scrollbarPosition", false);
 // Clipboard only supports text/plain
 pref("clipboard.plainTextOnly", false);
 
-#ifdef XP_WIN
+#if defined(XP_WIN) || defined(MOZ_WIDGET_GTK)
   // Setting false you can disable 4th button and/or 5th button of your mouse.
   // 4th button is typically mapped to "Back" and 5th button is typically mapped
   // to "Forward" button.

widget/gtk/nsWindow.cpp

@@ -3027,9 +3027,15 @@ void nsWindow::OnButtonPressEvent(GdkEventButton* aEvent) {
       return;
     // Map buttons 8-9 to back/forward
     case 8:
+      if (!Preferences::GetBool("mousebutton.4th.enabled", true)) {
+        return;
+      }
       DispatchCommandEvent(nsGkAtoms::Back);
       return;
     case 9:
+      if (!Preferences::GetBool("mousebutton.5th.enabled", true)) {
+        return;
+      }
       DispatchCommandEvent(nsGkAtoms::Forward);
       return;
     default: