layout: Offer to enable hot corner after repeated attempts to trigger it

Whether people love or hate the hot corner depends in large extents
on hardware sensitivity and habits, which is hard to get right
universally. So bite the bullet and add an option to enable or
disable hot corners ...

https://bugzilla.gnome.org/show_bug.cgi?id=688320

data/org.gnome.shell.gschema.xml.in

@@ -38,6 +38,14 @@
         load all extensions regardless of the versions they claim to support.
       </description>
     </key>
+    <key name="enable-hot-corners" type="b">
+      <default>false</default>
+      <summary>Enable hot corners</summary>
+      <description>
+        If true, the overview can be accessed by moving the mouse to the
+        top-left corner.
+      </description>
+    </key>
     <key name="favorite-apps" type="as">
       <default>[ 'epiphany.desktop', 'evolution.desktop', 'rhythmbox.desktop', 'shotwell.desktop', 'org.gnome.Nautilus.desktop', 'org.gnome.Software.desktop' ]</default>
       <summary>List of desktop file IDs for favorite applications</summary>

js/ui/layout.js

@@ -6,6 +6,7 @@ const Signals = imports.signals;
 const Background = imports.ui.background;
 const BackgroundMenu = imports.ui.backgroundMenu;
 const LoginManager = imports.misc.loginManager;
+const MessageTray = imports.ui.messageTray;
 
 const DND = imports.ui.dnd;
 const Main = imports.ui.main;
@@ -19,6 +20,11 @@ var BACKGROUND_FADE_ANIMATION_TIME = 1.0;
 var HOT_CORNER_PRESSURE_THRESHOLD = 100; // pixels
 var HOT_CORNER_PRESSURE_TIMEOUT = 1000; // ms
 
+// trigger attempts after which we offer to enable the corner
+var HOT_CORNER_TRIGGER_THRESHOLD = 2;
+// maximum time between trigger attempts
+var HOT_CORNER_TRIGGER_TIMEOUT = 1; // s
+
 function isPopupMetaWindow(actor) {
     switch(actor.meta_window.get_window_type()) {
     case Meta.WindowType.DROPDOWN_MENU:
@@ -1091,6 +1097,11 @@ var HotCorner = class HotCorner {
         // multiple times due to an accidental jitter.
         this._entered = false;
 
+        // Count how often the disabled hot corner would have been triggered
+        // in a particular interval; this is used to offer the user to enable
+        // the corner when passing a threshold.
+        this._triggerCount = 0;
+
         this._monitor = monitor;
 
         this._x = x;
@@ -1234,10 +1245,48 @@ var HotCorner = class HotCorner {
         this._animRipple(this._ripple3, 0.35,  1.0,   0.0,   0.3,     1);
     }
 
+    _resetTriggerTimeout() {
+        if (this._triggerTimeoutId)
+            GLib.source_remove(this._triggerTimeoutId);
+
+        this._triggerTimeoutId = GLib.timeout_add_seconds(
+            GLib.PRIORITY_DEFAULT,
+            HOT_CORNER_TRIGGER_TIMEOUT,
+            () => {
+                this._triggerCount = 0;
+
+                this._triggerTimeoutId = 0;
+                return GLib.SOURCE_REMOVE;
+            });
+    }
+
+    _showEnableNotification() {
+        let source = new MessageTray.SystemNotificationSource();
+        Main.messageTray.add(source);
+        let notification = new MessageTray.Notification(source,
+            _('Are you trying to access the activities overview?'),
+            _('GNOME can open the overview every time the pointer is moved to the corner'));
+        notification.setTransient(true);
+        notification.addAction(_('Enable corner gesture'), () => {
+            global.settings.set_boolean('enable-hot-corners', true);
+        });
+        source.notify(notification);
+}
+
     _toggleOverview() {
         if (this._monitor.inFullscreen && !Main.overview.visible)
             return;
 
+        if (!global.settings.get_boolean('enable-hot-corners')) {
+            this._resetTriggerTimeout();
+            this._triggerCount++;
+
+            if (this._triggerCount >= HOT_CORNER_TRIGGER_THRESHOLD)
+                this._showEnableNotification();
+
+            return;
+        }
+
         if (Main.overview.shouldToggleByCornerOrButton()) {
             this._rippleAnimation();
             Main.overview.toggle();