Preserve fullscreen when exiting video fullscreen

Fixes #2778

qutebrowser/browser/webengine/webenginetab.py

@@ -615,6 +615,7 @@ class WebEngineTab(browsertab.AbstractTab):
 
     def shutdown(self):
         self.shutting_down.emit()
+        self.action.exit_fullscreen()
         if qtutils.version_check('5.8', exact=True):
             # WORKAROUND for
             # https://bugreports.qt.io/browse/QTBUG-58563

qutebrowser/mainwindow/mainwindow.py

@@ -128,6 +128,8 @@ class MainWindow(QWidget):
         _commandrunner: The main CommandRunner instance.
         _overlays: Widgets shown as overlay for the current webpage.
         _private: Whether the window is in private browsing mode.
+        _restore_fullscreen: Whether to restore the fullscreen after leaving
+            a video fullscreen.
     """
 
     def __init__(self, *, private, geometry=None, parent=None):
@@ -217,6 +219,8 @@ class MainWindow(QWidget):
 
         objreg.get("app").new_window.emit(self)
 
+        self._restore_fullscreen = False
+
     def _init_geometry(self, geometry):
         """Initialize the window geometry or load it from disk."""
         if geometry is not None:
@@ -483,9 +487,14 @@ class MainWindow(QWidget):
     @pyqtSlot(bool)
     def _on_fullscreen_requested(self, on):
         if on:
+            self._restore_fullscreen = self.isFullScreen()
             self.showFullScreen()
-        else:
+        elif not self._restore_fullscreen:
             self.showNormal()
+        else:
+            self._restore_fullscreen = self.isFullScreen()
+        log.misc.debug('on: {}, restore fullscreen: {}'
+                       .format(on, self._restore_fullscreen))
 
     @cmdutils.register(instance='main-window', scope='window')
     @pyqtSlot()