Add a fullscreen notification overlay

From the spec:

  User agents should ensure, e.g. by means of an overlay, that the end user is
  aware something is displayed fullscreen. User agents should provide a means of
  exiting fullscreen that always works and advertise this to the user. This is
  to prevent a site from spoofing the end user by recreating the user agent or
  even operating system environment when fullscreen.

https://fullscreen.spec.whatwg.org/#security-and-privacy-considerations

qutebrowser/browser/webengine/webenginetab.py

@@ -38,6 +38,7 @@ from qutebrowser.browser import browsertab, mouse, shared
 from qutebrowser.browser.webengine import (webview, webengineelem, tabhistory,
                                            interceptor, webenginequtescheme,
                                            webenginedownloads)
+from qutebrowser.misc import miscwidgets
 from qutebrowser.utils import (usertypes, qtutils, log, javascript, utils,
                                objreg, jinja)
 
@@ -654,9 +655,13 @@ class WebEngineTab(browsertab.AbstractTab):
 
     @pyqtSlot('QWebEngineFullScreenRequest')
     def _on_fullscreen_requested(self, request):
-        # FIXME:qtwebengine do we want a setting to disallow this?
         request.accept()
-        self.fullscreen_requested.emit(request.toggleOn())
+        on = request.toggleOn()
+        self.fullscreen_requested.emit(on)
+        if on:
+            notification = miscwidgets.FullscreenNotification(self)
+            notification.show()
+            notification.set_timeout(3000)
 
     def _connect_signals(self):
         view = self._widget

qutebrowser/misc/miscwidgets.py

@@ -19,12 +19,12 @@
 
 """Misc. widgets used at different places."""
 
-from PyQt5.QtCore import pyqtSlot, pyqtSignal, Qt, QSize
+from PyQt5.QtCore import pyqtSlot, pyqtSignal, Qt, QSize, QTimer
 from PyQt5.QtWidgets import (QLineEdit, QWidget, QHBoxLayout, QLabel,
-                             QStyleOption, QStyle, QLayout)
+                             QStyleOption, QStyle, QLayout, QApplication)
 from PyQt5.QtGui import QValidator, QPainter
 
-from qutebrowser.utils import utils
+from qutebrowser.utils import utils, objreg
 from qutebrowser.misc import cmdhistory
 
 
@@ -264,3 +264,42 @@ class WrapperLayout(QLayout):
     def unwrap(self):
         self._widget.setParent(None)
         self._widget.deleteLater()
+
+
+class FullscreenNotification(QLabel):
+
+    """A label telling the user this page is now fullscreen."""
+
+    def __init__(self, parent=None):
+        super().__init__(parent)
+        self.setStyleSheet("""
+            background-color: rgba(50, 50, 50, 80%);
+            color: white;
+            border-radius: 20px;
+            padding: 30px;
+        """)
+
+        key_config = objreg.get('key-config')
+        all_bindings = key_config.get_reverse_bindings_for('normal')
+        bindings = all_bindings.get('fullscreen --leave')
+        if bindings:
+            key = bindings[0]
+            if utils.is_special_key(key):
+                key = key.strip('<>').capitalize()
+            self.setText("Press {} to exit fullscreen.".format(key))
+        else:
+            self.setText("Page is now fullscreen.")
+
+        self.resize(self.sizeHint())
+        geom = QApplication.desktop().screenGeometry(self)
+        self.move((geom.width() - self.sizeHint().width()) / 2, 30)
+
+    def set_timeout(self, timeout):
+        """Hide the widget after the given timeout."""
+        QTimer.singleShot(timeout, self._on_timeout)
+
+    @pyqtSlot()
+    def _on_timeout(self):
+        """Hide and delete the widget."""
+        self.hide()
+        self.deleteLater()

tests/unit/misc/test_miscwidgets.py

@@ -100,3 +100,26 @@ class TestWrapperLayout:
     def test_wrapped(self, container):
         assert container.wrapped.parent() is container
         assert container.focusProxy() is container.wrapped
+
+
+class TestFullscreenNotification:
+
+    @pytest.mark.parametrize('bindings, text', [
+        ({'<escape>': 'fullscreen --leave'},
+            "Press Escape to exit fullscreen."),
+        ({'<escape>': 'fullscreen'}, "Page is now fullscreen."),
+        ({'a': 'fullscreen --leave'}, "Press a to exit fullscreen."),
+        ({}, "Page is now fullscreen."),
+    ])
+    def test_text(self, qtbot, key_config_stub, bindings, text):
+        key_config_stub.set_bindings_for('normal', bindings)
+        w = miscwidgets.FullscreenNotification()
+        qtbot.add_widget(w)
+        assert w.text() == text
+
+    def test_timeout(self, qtbot, key_config_stub):
+        key_config_stub.set_bindings_for('normal', {})
+        w = miscwidgets.FullscreenNotification()
+        qtbot.add_widget(w)
+        with qtbot.waitSignal(w.destroyed):
+            w.set_timeout(1)