diff --git a/setup/qt5-migrate.py b/setup/qt5-migrate.py
index e957e95e5a..327c6beb43 100644
--- a/setup/qt5-migrate.py
+++ b/setup/qt5-migrate.py
@@ -6,7 +6,8 @@ from __future__ import (unicode_literals, division, absolute_import,
 __license__ = 'GPL v3'
 __copyright__ = '2014, Kovid Goyal <kovid at kovidgoyal.net>'
 
-# QT5XX: See why mouse wheel is not working in diff view
+# QT5XX: Port def wheelEvent() (orientation() and delta() to be replaced by
+# angleDelta())
 
 # QT5XX: Delete this file after migration is completed
 
diff --git a/src/calibre/gui2/tweak_book/diff/view.py b/src/calibre/gui2/tweak_book/diff/view.py
index 1693d0244b..1f8e0d1a8d 100644
--- a/src/calibre/gui2/tweak_book/diff/view.py
+++ b/src/calibre/gui2/tweak_book/diff/view.py
@@ -381,7 +381,7 @@ class TextBrowser(PlainTextEdit):  # {{{
             painter.drawLine(0, bottom - 1, w, bottom - 1)
 
     def wheelEvent(self, ev):
-        if ev.orientation() == Qt.Vertical:
+        if ev.angleDelta().x() == 0:
             self.wheel_event.emit(ev)
         else:
             return PlainTextEdit.wheelEvent(self, ev)
@@ -488,7 +488,7 @@ class DiffSplitHandle(QSplitterHandle):  # {{{
         return ans
 
     def wheelEvent(self, ev):
-        if ev.orientation() == Qt.Vertical:
+        if ev.angleDelta().x() == 0:
             self.wheel_event.emit(ev)
         else:
             return QSplitterHandle.wheelEvent(self, ev)