The qt backend is not working on mac big sur as reported in#18954.
It is fixed in qt 5.15.2 (https://codereview.qt-project.org/c/qt/qtbase/+/322228) and in the coming 5.12.11 (https://codereview.qt-project.org/c/qt/qtbase/+/322507) but setting this environment variable is necessary for qt <5.15.2.

Even if in a ideal wordmatplotlib shouldn't be setting this variable environment, this is pragmatic and very convenient fix!

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SetQT_MAC_WANTS_LAYER='1' on mac to fix issue with the qt backend …

0065d4b

…on mac big sur.

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Jan 21, 2021

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Fix linting.

ccfdf5a

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jklymak commentedJan 21, 2021

Seems reasonable.@dstansby you mentioned having Big Sur? (ahem, I suppose I should upgrade one of my machines one of these days, but...)

jklymak added OS: Apple GUI: Qt labels

Jan 21, 2021

jklymak added this to thev3.4.0 milestone

Jan 21, 2021

tacaswell reviewed

Jan 22, 2021

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tacaswell commentedJan 22, 2021

On one hand, this seem to have only moderate down side, but if it is fixed in newer versions of (py?)Qt it seems to be a moot point?

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dstansby commentedJan 22, 2021

I'm struggling to reproduce the original bug - I think it's worth discussing at#18954 (comment) before going further with this.

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dstansby commentedJan 22, 2021

I can reproduce the original issue, and confirm that setting this environment variable fixes it.

Set environment variable only on macos big sur and if it isn't alread…

76e8260

…y set.

anntzer reviewed

Jan 23, 2021

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dstansby commentedJan 24, 2021

I was wondering if this would break older versions of OSX/macos, but#18134 says QT5 requires > 10.13, andhttps://bugreports.qt.io/browse/QTBUG-87014 seems to say that setting this should be fine going back to 10.13, so this looks good to me.

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Jan 24, 2021

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dstansby commentedJan 24, 2021

Does anyone have thoughts on backporting this to 3.3.x? Seems reasonable to me.

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ericpre commentedJan 24, 2021

I was wondering if this would break older versions of OSX/macos, but#18134 says QT5 requires > 10.13, andhttps://bugreports.qt.io/browse/QTBUG-87014 seems to say that setting this should be fine going back to 10.13, so this looks good to me.

76e8260 adds an additional conditionLooseVersion(platform.mac_ver()[0]) >= LooseVersion("10.16"), so this should be fine in any case!

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Jan 24, 2021

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lib/matplotlib/backends/qt_compat.py

		# Fixes issues with Big Sur
		# https://bugreports.qt.io/browse/QTBUG-87014, fixed in qt 5.15.2
		if (sys.platform == 'darwin' and
		LooseVersion(platform.mac_ver()[0]) >= LooseVersion("10.16") and

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ericpreJan 24, 2021•
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I don't know if this should be a>= or== and I don't know if we can even know how is it going to be with the next MacOS release...

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tacaswell commentedJan 28, 2021

The choices are:

strict equality it may break on the next verssion of OSX if this ends up being a required flag and we have to go through this again.
= and it breaks on the next version of OSX because Qt / OSX change to make this aharmful env

Qt/pyqt is generally pretty good about back-compatibility / not breaking users so if we are going to do this, the version filtering seems right.

My main hang up about this is still that libraries should probably not be setting envs (as that is users / application concerns).

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ericpre commentedJan 28, 2021

My main hang up about this is still that libraries should probably not be setting envs (as that is users / application concerns).

I agree that we shouldn't be doing this but in practise this is the best and less intrusive solution I know or I can think of. If theQT_MAC_WANTS_LAYER environment variable is already set, it will not be changed.
Another alternative would be to simply document it and let users find it on their own. I think that there are a non negligible numbers of users who don't know how to set variable environment (and are most likely not interested to figure it out) and they shouldn't have to do that, because there is no point in wasting time on this.