Don't use pixelDelta() on X11.

anntzer · anntzer · commit 6d0c9a877e9d · 2021-10-03T13:31:57.000+02:00
pixelDelta() is explicitly documented as "unreliable" and not-to-be-used on X11 (https://doc.qt.io/qt-5/qwheelevent.html#pixelDelta); locally, it reports the same value as angleDelta, but because it isn't scaled, the value is 120x too big. So add a check that forces the use of angleDelta() on X11 (see also https://doc.qt.io/qt-6/qguiapplication.html#platformName-prop).
diff --git a/lib/matplotlib/backends/backend_qt.py b/lib/matplotlib/backends/backend_qt.py
@@ -312,8 +312,10 @@ def mouseReleaseEvent(self, event):
 
     def wheelEvent(self, event):
         x, y = self.mouseEventCoords(self._get_position(event))
-        # from QWheelEvent::delta doc
-        if event.pixelDelta().x() == 0 and event.pixelDelta().y() == 0:
+        # from QWheelEvent::pixelDelta doc: pixelDelta is sometimes not
+        # provided (`isNull()`) and is unreliable on X11 ("xcb").
+        if (event.pixelDelta().isNull()
+                or QtWidgets.QApplication.instance().platformName() == "xcb"):
             steps = event.angleDelta().y() / 120
         else:
             steps = event.pixelDelta().y()
