BUG: Scrolling pager in vsplit on Mac OSX tears.

On Mac OS X, scrolling the pager when it is split vertically causes the
help text to tear (i.e. the help text is unreadable).  This hack
attempts to make it a little better although the solution may not be
optimal at all.

IPython/frontend/qt/console/console_widget.py

@@ -172,6 +172,13 class ConsoleWidget(LoggingConfigurable, QtGui.QWidget):
         QtGui.QWidget.__init__(self, parent)
         LoggingConfigurable.__init__(self, **kw)
 
+        # While scrolling the pager on Mac OS X, it tears badly.  The
+        # NativeGesture is platform and perhaps build-specific hence
+        # we take adequate precautions here.
+        self._pager_scroll_events = [QtCore.QEvent.Wheel]
+        if hasattr(QtCore.QEvent, 'NativeGesture'):
+            self._pager_scroll_events.append(QtCore.QEvent.NativeGesture)
+
         # Create the layout and underlying text widget.
         layout = QtGui.QStackedLayout(self)
         layout.setContentsMargins(0, 0, 0, 0)
@@ -364,17 +371,11 class ConsoleWidget(LoggingConfigurable, QtGui.QWidget):
             QtGui.qApp.sendEvent(obj, QtGui.QDragLeaveEvent())
             return True
 
-        # Handle scrolling of the vsplit pager. This hack attempts to solve the
-        # problem of tearing of the pager window's help text on Mac OS X.  This
-        # happens with PySide and PyQt. This fix isn't perfect but makes the
-        # pager more usable.
-
-        # FIXME: this line, on qt 4.8.5, crashes b/c NativeGesture is not
-        # available.   Disabling for now, until we sort out a cleaner solution.
-        # See https://github.com/ipython/ipython/pull/1150 for details.
-        #elif etype in [QtCore.QEvent.Wheel, QtCore.QEvent.NativeGesture] and \
-
-        elif etype == QtCore.QEvent.Wheel and \
+        # Handle scrolling of the vsplit pager. This hack attempts to solve
+        # problems with tearing of the help text inside the pager window.  This
+        # happens only on Mac OS X with both PySide and PyQt. This fix isn't
+        # perfect but makes the pager more usable.
+        elif etype in self._pager_scroll_events and \
                 obj == self._page_control:
             self._page_control.repaint()
             return True