Reimplement wheelEvent of WebView to fix scrolling

Hiding scrollbars in 1bfbdd79e4 broke
scrolling because the WebKit implementation wants a scrollbar to do
scrolling.

We fix this by just handling the mouse wheel ourselves.

qutebrowser/config/configdata.py

@@ -277,6 +277,10 @@ DATA = OrderedDict([
         ('forward-unbound-keys',
          SettingValue(types.Bool(), 'false'),
          "Whether to forward unbound keys to the website in normal mode."),
+
+        ('scroll-amount',
+         SettingValue(types.Float(minval=0), '0.5'),
+         "How many pixels to scroll per 1/8 degree mousewheel delta."),
     )),
 
     ('tabbar', sect.KeyValue(

qutebrowser/widgets/webview.py

@@ -20,7 +20,7 @@
 import logging
 import functools
 
-from PyQt5.QtCore import pyqtSignal, pyqtSlot, Qt
+from PyQt5.QtCore import pyqtSignal, pyqtSlot, Qt, QPoint
 from PyQt5.QtWidgets import QApplication
 from PyQt5.QtWebKit import QWebSettings
 from PyQt5.QtWebKitWidgets import QWebView, QWebPage
@@ -526,3 +526,17 @@ class WebView(QWebView):
         self._mousepress_insertmode(e)
         self._mousepress_opentarget(e)
         return super().mousePressEvent(e)
+
+    def wheelEvent(self, e):
+        """Override wheelEvent because we handle scrolling ourselves.
+
+        WebKit's scrolling doesn't allow us to scroll while scrollbars are
+        enabled...
+        """
+        if not e.pixelDelta().isNull():
+            delta = QPoint(e.pixelDelta())
+        else:
+            delta = e.angleDelta() * config.get('input', 'scroll-amount')
+        frame = self.page_.currentFrame()
+        pos = frame.scrollPosition() - delta
+        frame.setScrollPosition(pos)