[PyQt] Occasionally getting QBuffer in QApplication.focusChanged slot

[Florian Bruhin]

I have a slot (decorated with @pyqtSlot('QWidget*', 'QWidget*')) which
is connected to QApplication::focusChanged:

https://github.com/The-Compiler/qutebrowser/blob/v0.6.2/qutebrowser/app.py#L154
https://github.com/The-Compiler/qutebrowser/blob/v0.6.2/qutebrowser/app.py#L342-L357

Now it seems sometimes that slot gets called with a QBuffer as
argument:

	Traceback (most recent call last):
	  File ".../qutebrowser/app.py", line 348, in on_focus_changed
		window = new.window()
	AttributeError: 'QBuffer' object has no attribute 'window'

I couldn't reproduce it myself so far, but I've seen this reported by
three people (with PyQt 5.5.1 / Qt 5.6).

I'll just add a type check which ignores this scenario, but I'm kind
of curious how this happens, so if anyone has an idea what's going on,
let me know!

Florian

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