Fix MDI area so that when scrolling over a sub-window, the scrolling …

…action does not also apply to the area as a whole.

glue/app/qt/mdi_area.py

@@ -83,10 +83,38 @@ def paintEvent(self, event):
         painter.drawText(rect, Qt.AlignHCenter | Qt.AlignVCenter,
                          "Drag Data To Plot")
 
+    def wheelEvent(self, event):
+
+        # NOTE: when a scroll wheel event happens on top of a GlueMdiSubWindow,
+        # we need to ignore it in GlueMdiArea to prevent the canvas from moving
+        # around. I couldn't find a clean way to do this with events, so instead
+        # in GlueMdiSubWindow I set a flag, _wheel_event, to indicate that a
+        # wheel event has happened in a subwindow, which means the next time
+        # the GlueMdiArea.wheelEvent gets called, we should ignore the wheel
+        # event.
+
+        any_subwindow_wheel = False
+
+        for window in self.subWindowList():
+            if getattr(window, '_wheel_event', None):
+                any_subwindow_wheel = True
+                window._wheel_event = None
+
+        if any_subwindow_wheel:
+            event.ignore()
+            return
+
+        super(GlueMdiArea, self).wheelEvent(event)
+
 
 class GlueMdiSubWindow(QtWidgets.QMdiSubWindow):
     closed = QtCore.Signal()
 
+    def wheelEvent(self, event):
+        # See NOTE in GlueMdiArea.wheelEvent
+        self._wheel_event = True
+        super(GlueMdiSubWindow, self).wheelEvent(event)
+
     def closeEvent(self, event):
         super(GlueMdiSubWindow, self).closeEvent(event)
         self.closed.emit()