I am using PyQT5 and VTK for displaying information, and have been using observers to register mouse click events through the interactor style. For some reason, when multiple buttons are clicked, the release event gets lost. I have modified one of the VTK QT examples to do this:

# Contributed by Eric E Monson

import sys

# noinspection PyUnresolvedReferences
import vtkmodules.vtkInteractionStyle
# noinspection PyUnresolvedReferences
import vtkmodules.vtkRenderingOpenGL2
from PyQt5 import QtCore, QtWidgets, QtGui
from PyQt5.QtWidgets import QApplication
from vtkmodules.vtkInteractionStyle import vtkInteractorStyleTrackballCamera,vtkInteractorStyleUser
from vtk.qt.QVTKRenderWindowInteractor import QVTKRenderWindowInteractor
from vtkmodules.vtkFiltersSources import vtkSphereSource
from vtkmodules.vtkRenderingCore import (
    vtkActor,
    vtkPolyDataMapper,
    vtkRenderer
)

class MyInteractorStyle(vtkInteractorStyleTrackballCamera):

    def __init__(self, parent=None):
        self.AddObserver('LeftButtonPressEvent', self.left_button_press_event)
        self.AddObserver('LeftButtonReleaseEvent', self.left_button_release_event)
        self.AddObserver('MiddleButtonPressEvent', self.middle_button_press_event)
        self.AddObserver('MiddleButtonReleaseEvent', self.middle_button_release_event)

    def middle_button_press_event(self, obj, event):
        print('Middle Button pressed')
        #self.OnMiddleButtonDown()
        return

    def middle_button_release_event(self, obj, event):
        print('Middle Button released')
        #self.OnMiddleButtonUp()
        return

    def left_button_press_event(self, obj, event):
        print('Left Button pressed')
        #self.OnLeftButtonDown()
        return

    def left_button_release_event(self, obj, event):
        print('Left Button released')
        #self.OnLeftButtonUp()
        return

class Ui_MainWindow(object):
    def setupUi(self, MainWindow):
        MainWindow.setObjectName("MainWindow")
        MainWindow.resize(603, 553)
        self.centralWidget = QtWidgets.QWidget(MainWindow)
        self.gridlayout = QtWidgets.QGridLayout(self.centralWidget)
        self.vtkWidget = QVTKRenderWindowInteractor(self.centralWidget)
        self.gridlayout.addWidget(self.vtkWidget, 0, 0, 1, 1)
        MainWindow.setCentralWidget(self.centralWidget)


class SimpleView(QtWidgets.QMainWindow):

    def __init__(self, parent=None):
        QtWidgets.QMainWindow.__init__(self, parent)
        self.ui = Ui_MainWindow()
        self.ui.setupUi(self)
        self.ren = vtkRenderer()
        self.ui.vtkWidget.GetRenderWindow().AddRenderer(self.ren)
        self.iren = self.ui.vtkWidget.GetRenderWindow().GetInteractor()
        self.iren.SetInteractorStyle(MyInteractorStyle())

        # Create source
        source = vtkSphereSource()
        source.SetCenter(0, 0, 0)
        source.SetRadius(5.0)

        # Create a mapper
        mapper = vtkPolyDataMapper()
        mapper.SetInputConnection(source.GetOutputPort())

        # Create an actor
        actor = vtkActor()
        actor.SetMapper(mapper)

        self.ren.AddActor(actor)


if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = SimpleView()
    window.show()
    window.iren.Initialize()  # Need this line to actually show the render inside Qt
    sys.exit(app.exec_())

Performing the following operations:

1. middle press and hold
2. left press
3. left release
4. middle release

Yields the following output:

1. Middle Button pressed
2. Left Button pressed
3. Left Button released
4. Left Button released

The last event is clearly incorrect. Any permutation of pressing and holding seems to do this. It seems as though the event registering is not working, as the last button pressed is what gets ‘released’. This behavior does not persist when using VTK outside of pyqt, leading me to believe there is an issue with the QVTKRenderWindowInteractor.