[gnome-shell] magnifier.js: Fix zoom juddering

[Florian Müllner <fmuellner src gnome org>]

commit 94101e8bb88e86e8ae9f07ae4e0e4fc568441a2d
Author: Daniel van Vugt <daniel van vugt canonical com>
Date:   Thu May 10 18:13:49 2018 +0800

    magnifier.js: Fix zoom juddering
    
    Make Zoom respond to the mouse silky-smoothly.
    
    It was previously hard-coded to: 1000/50 = 20 FPS.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=682013
    https://launchpad.net/bugs/1691675

 js/ui/magnifier.js | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
---
diff --git a/js/ui/magnifier.js b/js/ui/magnifier.js
index b752b127a..3103630ca 100644
--- a/js/ui/magnifier.js
+++ b/js/ui/magnifier.js
@@ -19,7 +19,6 @@ const MagnifierDBus = imports.ui.magnifierDBus;
 const Params = imports.misc.params;
 const PointerWatcher = imports.ui.pointerWatcher;
 
-var MOUSE_POLL_FREQUENCY = 50;
 var CROSSHAIRS_CLIP_SIZE = [100, 100];
 var NO_CHANGE = 0.0;
 
@@ -152,8 +151,10 @@ var Magnifier = new Lang.Class({
      * Turn on mouse tracking, if not already doing so.
      */
     startTrackingMouse() {
-        if (!this._pointerWatch)
-            this._pointerWatch = PointerWatcher.getPointerWatcher().addWatch(MOUSE_POLL_FREQUENCY, 
this.scrollToMousePos.bind(this));
+        if (!this._pointerWatch) {
+            let interval = 1000 / Clutter.get_default_frame_rate();
+            this._pointerWatch = PointerWatcher.getPointerWatcher().addWatch(interval, 
this.scrollToMousePos.bind(this));
+        }
     },
 
     /**