FWICT, the problem seems to be mixed signals across multiple figures. The test in question has a second portion (for Agg-based backends only, which is why Cairo ones don't fail) which opens a figure to lock in the backend, then attempts to switch backends and confirm that it fails. This occurs before running the main test of opening a window, and closing it after the first draw event. In that case, the Matplotlib side does correctly close the Qt window, but the closing signal emitted through Qt seems to trigger the callback from the first figure. The first figure manager/canvas doesn't have any callbacks connected to it to print the CloseEvent, so the test fails.

Using this patch to run a copy of the test:

diff --git a/lib/matplotlib/backends/backend_qt.py b/lib/matplotlib/backends/backend_qt.py
index faeb0d6bb7..4cfa294c6e 100644
--- a/lib/matplotlib/backends/backend_qt.py
+++ b/lib/matplotlib/backends/backend_qt.py
@@ -501,6 +501,7 @@ class MainWindow(QtWidgets.QMainWindow):
     closing = QtCore.Signal()

     def closeEvent(self, event):
+        print('closeEvent', self, event, self.closing)
         self.closing.emit()
         super().closeEvent(event)

@@ -522,9 +523,11 @@ class FigureManagerQT(FigureManagerBase):
     def __init__(self, canvas, num):
         self.window = MainWindow()
         super().__init__(canvas, num)
-        self.window.closing.connect(
-            # The lambda prevents the event from being immediately gc'd.
-            lambda: CloseEvent("close_event", self.canvas)._process())
+        print('Creating new figure manager', self, canvas, num, self.window, self.window.closing)
+        def foo():
+            print('close event is happening!', self, self.canvas, self.window)
+            CloseEvent("close_event", self.canvas)._process()
+        self.window.closing.connect(foo)
         self.window.closing.connect(self._widgetclosed)

         if sys.platform != "darwin":
@@ -604,6 +607,7 @@ class FigureManagerQT(FigureManagerBase):
             self.window.raise_()

     def destroy(self, *args):
+        print('FigureManagerQT.destroy', self, self.window)
         # check for qApp first, as PySide deletes it in its atexit handler
         if QtWidgets.QApplication.instance() is None:
             return

outputs:

Creating new figure manager <matplotlib.backends.backend_qt.FigureManagerQT object at 0x7fc90fe38a60> <matplotlib.backends.backend_qtagg.FigureCanvasQTAgg(0x560b8d915020) at 0x7fc90fcc1f80> 1 <matplotlib.backends.backend_qt.MainWindow(0x560b8d91b750) at 0x7fc90fcc24c0> <PySide6.QtCore.SignalInstance closing() at 0x7fc90fffa1b0>
FigureManagerQT.destroy <matplotlib.backends.backend_qt.FigureManagerQT object at 0x7fc90fe38a60> <matplotlib.backends.backend_qt.MainWindow(0x560b8d91b750) at 0x7fc90fcc24c0>
closeEvent <matplotlib.backends.backend_qt.MainWindow(0x560b8d91b750) at 0x7fc90fcc24c0> <PySide6.QtCore.QEvent(QEvent::Close)> <PySide6.QtCore.SignalInstance closing() at 0x7fc90fffa1b0>
close event is happening! <matplotlib.backends.backend_qt.FigureManagerQT object at 0x7fc90fe38a60> <matplotlib.backends.backend_qtagg.FigureCanvasQTAgg(0x560b8d915020) at 0x7fc90fcc1f80> <matplotlib.backends.backend_qt.MainWindow(0x560b8d91b750) at 0x7fc90fcc24c0>

Creating new figure manager <matplotlib.backends.backend_qt.FigureManagerQT object at 0x7fc90d581900> <matplotlib.backends.backend_qtagg.FigureCanvasQTAgg(0x560b8d9b5ea0) at 0x7fc90d590340> 1 <matplotlib.backends.backend_qt.MainWindow(0x560b8d9e4010) at 0x7fc90d7dfd00> <PySide6.QtCore.SignalInstance closing() at 0x7fc90d715830>
FigureManagerQT.destroy <matplotlib.backends.backend_qt.FigureManagerQT object at 0x7fc90d581900> <matplotlib.backends.backend_qt.MainWindow(0x560b8d9e4010) at 0x7fc90d7dfd00>
closeEvent <matplotlib.backends.backend_qt.MainWindow(0x560b8d9e4010) at 0x7fc90d7dfd00> <PySide6.QtCore.QEvent(QEvent::Close)> <PySide6.QtCore.SignalInstance closing() at 0x7fc90d715830>
close event is happening! <matplotlib.backends.backend_qt.FigureManagerQT object at 0x7fc90fe38a60> <matplotlib.backends.backend_qtagg.FigureCanvasQTAgg(0x560b8d915020) at 0x7fc90fcc1f80> <matplotlib.backends.backend_qt.MainWindow(0x560b8d91b750) at 0x7fc90fcc24c0>

The second figure has a figure.canvas.manager of 0x7fc90d581900 and a figure.canvas.manager.window.closing instance of 0x7fc90d715830, and you can see that closeEvent receives it for that manager (so the correct window should be closed) and the same closing instance. But then when we get the callback for that signal, it's using the manager for the first window.

Uh oh!

[TST] Qt/Pyside 6.5.1 dependency test failures #25988

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