WIP, ENH: Automatic theme switching on macOS

[GuillaumeFavelier]

This PR comes from #10565 (comment) and adds support for automatic theme switching on macOS. It is a draft until it has been tested on macOS.

How to test?

On a macOS machine, plot any 3d figure (alignment, stc, coreg, ...etc) then switch the system theme.

Expected result

The window, the widgets and the icons should follow the new theme.

---

Closes #9182

[hoechenberger]

Related:
Yesterday's release of darkdetect allows automatic theme switching detection on Windows and Linux:

https://github.com/albertosottile/darkdetect/releases/tag/v0.6.0

[cbrnr]

Yesterday's release of darkdetect allows automatic theme switching detection on Windows and Linux:

👀 – this is great! But Qt application still don't change to dark (native look) on Windows and Linux unless stylesheets are used, right?

[hoechenberger]

But Qt application still don't change to dark (native look) on Windows and Linux unless stylesheets are used, right?

I'm afraid so

[GuillaumeFavelier]

automatic theme switching detection on Windows and Linux:

Too bad it's only for Windows and Linux, it would be nice to use the same approach for all systems if possible.
I looked at the code for the linux part, is it only for Gnome desktop? Anyway, I'll give it a try locally, thanks for the heads-up 👍

[cbrnr]

Detection/switching on macOS has worked for a long time I thought? And yes, it seems to be GNOME-specific.

[hoechenberger]

Detection/switching on macOS has worked for a long time I thought? And yes, it seems to be GNOME-specific.

I suppose this PR is about switching without restarting the app.

[cbrnr]

This has worked on macOS for years.

[hoechenberger]

This has worked on macOS for years.

But not with Qt out of the box…

[hoechenberger]

This has worked on macOS for years.

But not with Qt out of the box…

MWE:

from PyQt5.QtWidgets import QApplication, QWidget, QPushButton
import sys

class PushButton(QWidget):
    def __init__(self):
        super(PushButton,self).__init__()
        self.initUI()

    def initUI(self):
        self.setWindowTitle("PushButton")
        self.setGeometry(400,400,300,260)
        self.closeButton = QPushButton(self)
        self.closeButton.setText("Close")
        self.closeButton.clicked.connect(self.close)
        self.closeButton.move(100,100)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    ex = PushButton()
    ex.show()
    sys.exit(app.exec_())

Switching between light and dark only affects the title bar:

Screen.Recording.2022-05-24.at.11.47.11.mov

[cbrnr]

Works for me. I've used this for years with MNELAB. macOS is the only platform where this works, Windows and Linux require a workaround (e.g. with a listener thread constantly polling for the theme).

mwe.mov

[hoechenberger]

Son of a gun, you're right – seems the only reason it didn't work for me here is because I used an x86 Python install on my M1 Mac. With a native arm64 build, it works as in your video.

[GuillaumeFavelier]

Well in your case, you use the default 'dark style' macOS palette but this PR is about applying the one from qdarkstyle :)

[GuillaumeFavelier]

darkdetect offers to detect theme change on Windows and Linux with their new listener but this PR relies on Qt PaletteChange event for that on macOS. That's why I say:

it would be nice to use the same approach for all systems if possible.

[GuillaumeFavelier]

There is also the option to use the darkdetect+qdarkstyle combination only on Windows and Linux (the operating systems that do not change the Qt palette automatically) and just update the icon theme when necessary for macOS, similarly to mnelab.

[cbrnr]

To me, it works as it should only on macOS. It would be great if Qt added support for theme changes on Windows and Linux via the PaletteChange event as well. I prefer native look and feel over styled widgets.

[GuillaumeFavelier]

It would be great if Qt added support for theme changes on Windows and Linux via the PaletteChange event as well.

I agree, anything consistent across the main OSes/desktops would be great. But it seems that darkdetect tries to fill the gap 👍

[cbrnr]

I agree, anything consistent across the main OSes/desktops would be great. But it seems that darkdetect tries to fill the gap 👍

Yep! Not only something consistent, but Qt should support native light/dark modes on Windows and Linux too. On Windows and Linux the app needs to use stylesheets, which doesn't look like native apps.

[hoechenberger]

There is also the option to use the darkdetect+qdarkstyle combination only on Windows and Linux (the operating systems that do not change the Qt palette automatically) and just update the icon theme when necessary for macOS, similarly to mnelab.

Yes I would love that, but @larsoner was opposed. I don't think our custom dark theme looks native on macOS. It feels a little strange.

But let's address this in a different issue or PR

[GuillaumeFavelier]

Okay, I did not remember all the details of the discussion back then.

Does the PR work in its current state by the way, anybody tried?

[larsoner]

Yes I would love that, but @larsoner was opposed. I don't think our custom dark theme looks native on macOS. It feels a little strange.

I don't remember why I was opposed -- but in any case, currently qdarkstyle isn't really used on macOS for dark stylesheets when in dark mode, at least on PyQt6 where dark theming is correct. And I think for PyQt5 it overrides as little as possible already. So in any case, +1 for using native light/dark styling where they actually work from me.

There are cases where I think we can't get around using qdarkstyle, though, like when you ask for dark theme but your system is currently in light mode. Maybe this is what you're thinking of experiencing @hoechenberger ?

As an aside, I looked for quite some time to see if there is a way to tell Qt "even though my system is in light mode, use the native dark mode Qt styling" but I couldn't find a way to do it. They don't just apply an internal stylesheet to do it, it's coded in a class somehow somewhere IIRC using private attributes/methods so I didn't see a way we could use them.

[larsoner]

I think this won't quite do what I'd expect -- if I pass theme='light' when instantiating a brain, I would expect it not to change to dark when the system switches. So the user-selected option (light, dark, auto) needs to be stored on instantiation, and switching done only when it's auto.

[GuillaumeFavelier]

Oh yeah good idea!

[GuillaumeFavelier]

As an aside, I looked for quite some time to see if there is a way to tell Qt "even though my system is in light mode, use the native dark mode Qt styling" but I couldn't find a way to do it. They don't just apply an internal stylesheet to do it, it's coded in a class somehow somewhere IIRC using private attributes/methods so I didn't see a way we could use them.

Could it be the macOS system itself that changes the Qt palette somehow? It would explain why we need to filter PaletteChange event on macOS to update the icons. In this scenario, it's even possible that the stylesheet is stored on the macOS side. I am just guessing here of course, I did not investigate.

[larsoner]

Could it be the macOS system itself that changes the Qt palette somehow?

Could be, with stuff like:

https://github.com/qt/qtbase/blob/067b53864112c084587fa9a507eb4bde3d50a6e1/src/widgets/widgets/qtoolbar.cpp#L28-L30
https://github.com/qt/qtbase/tree/067b53864112c084587fa9a507eb4bde3d50a6e1/src/plugins/platforms/cocoa

etc. So maybe there is some way for us to dig into the cocoa platform plugin and say, in the case that a user is in light mode but requests dark mode, "cocoa, restyle this as dark mode" after a light-mode window is been created (or tell it to use dark mode before it creates a light mode one, but this seems harder). But we're getting pretty deep in the internals of Qt here...

What might be a low effort possible solution would be to post a question like this to the Qt forums somewhere, and hope someone knows how to do it. If the solution is easy, we implement it; if nobody responds, if someone is bored they can look into the Qt internals to figure out if there's a way to tell Cocoa to restyle (in the same way that the user changing their OS mode changes things); and until then we just use qdarkstyle and live with the blue.

[cbrnr]

@larsoner if you post a question in the forum please add a link to the thread here. I'm very interested to see if this is possible. Ideally, this should be added to Qt directly instead of rolling our own workaround/fix. And not just the macOS dark app in system light mode, but also support for native light/dark mode on Windows and Linux.

[larsoner]

I'll let someone else post when they're motivated :)

[cbrnr]

I'm motivated but I need to find the time to do it... Let's see. I could imagine that there are tons of threads about light/dark mode switching on Windows/Linux already, but I haven't even searched the forum properly.

[hoechenberger]

My 2 cts:

I don't see any reason to support selecting a different theme than what is currently set system-wide, as long as we allow automated switching upon system-wide theme change. It's so trivially easy to change the theme on macOS – for me, it means right-clicking on an icon in the menu bar. I'm not against having this functionality, but if it causes pain and more complicated code (does it? I'm not sure?), I'd say, just don't support it and tell users about that neat tool that I'm using, NightOwl 🦉

It even allows you to always start certain apps in light or dark mode, independent from the global system setting.

[cbrnr]

I agree with @hoechenberger. The priority should be to support automatic switching on all three OSs, preferably using a native styling (as opposed to stylesheets).

[larsoner]

It sounds like we might just wait on this one (?) and even if we don't, will try to avoid having qdarkstyle set the stylesheet where possible. So I'll close for now!