Qt: Set QStyleHints colorScheme property properly

[chaoticgd]

Description of Changes

Sets the colorScheme property of the qApp->styleHint() object whenever the theme is changed.

We're still using a palette heuristic to figure out if it's a light or dark theme since in some cases qApp->styleHints()->colourScheme() will return the wrong result (I've added comments explaining this in more detail).

This also removes some custom logic for detecting theme changes on macOS because it was conflicting with Qt's platform code (which now handles this automatically) which was causing an infinite loop.

Rationale behind Changes

This lets Qt know that we're using a light or dark theme, which will allow it to do a better job at drawing certain widgets. For example before checkboxes looked like this (with a dark OS theme set):

After these changes they look like this:

Suggested Testing Steps

Test on Windows and macOS with the native themes. Try changing between light and dark themes in the settings window, and try changing between light, dark and high contrast themes in the OS.

Did you use AI to help find, test, or implement this issue or feature?

No.

[chaoticgd]

Oh, macOS is still on an old Qt version.

[chaoticgd]

I could add a fallback, but there's been some talk of updating the version of Qt used on macOS, so I'll just leave it for now until some other members of the team decide what to do.

[chaoticgd]

Added the fallback.

[kamfretoz]

Looks good on Arch Linux KDE

[JordanTheToaster]

Appears to make the icons in Classic Windows and Native theme under Windows 10 washed out.

[chaoticgd]

Appears to make the icons in Classic Windows and Native theme under Windows 10 washed out.

This should be fixed now. It appears the problem was that some styles (e.g. the Windows Classic/Windows Vista style) just don't obey the new colorScheme property (but it's still reported to the application), so I've used the old palette heuristic to set said property in those cases. I don't love this, but since it's the same as the old behaviour it shouldn't make it worse at least.

[chaoticgd]

Removed the fallback again now that macOS has been updated. Needs testing again.

[JordanTheToaster]

Very good PR.

[kamfretoz]

Tasty.

[TheTechnician27]

• ✅ Palette based on whether the window text value is lighter or darker than the window value for the "fusion" theme seems more sensible than an arbitrary 0.5 text value cutoff.
• ✅ Changes to QtUtils besides that are just a formatting fix and adding a comment for another unrelated option.
• ✅ Color scheme property introduced in Qt 6.5 seems a lot better than trying to roll our own.
• ✅ Tested, and it fixes the outlines for me like it does for everyone else.
• ✅ Without an extensive read of the QStyleHints docs, I can't immediately think of a way this can be improved.

Aside: on line 202 of DropIndicators.cpp, it looks like a tab somehow got replaced in this PR with a bunch of spaces by accident.

[chaoticgd]

Looks like this PR has a regression where certain on platforms (e.g. macOS) it breaks live switching of OS themes. I'm unable to reproduce this as I think I don't have access to any environments where this feature worked properly previously.

So it looks like this will be blocked until someone else can get to the bottom of this.

Some wild ideas are:

• Calling styleHints()->unsetColorScheme.
• Moving where styleHints()->setColorScheme is called.
• Respond to the QStyleHints::colorSchemeChanged signal.

[chaoticgd]

I think the problem might be that I'm explicitly settings the colorScheme property rather than unsetting it for the native themes. But I remember that unsetting it makes it return wrong results in some cases, so then it can't be used directly (and Qt uses it directly internally...).

[chaoticgd]

Alright, I've changed how this works: Now I'm only setting the colorScheme property (including to Unknown where that makes sense, rather than Light or Dark), and not reading it, since if it's set to Unknown qApp->styleHints()->colorScheme() can return the wrong value (e.g. in the case of the Windows Classic theme), and if I never set it to Unknown (as I was doing before) we never get notified of OS colour scheme changes.

[chaoticgd]

Needs testing again. Sorry about that.

[SternXD]

dark/light mode switching works again after latest force push

I will say there is this error in System Console when you do switch them I don't think it's a blocker that's why I'm approving should probably be something to fix though eventually.

[TheTechnician27]

Lookin' good, Susan.

[F0bes]

Just for documentation sake, and so I remember..... I found that this conflicts with the cocoa theme change code and causes a feedback effect. Just waiting for this PR to remove that code.

[chaoticgd]

Removed the custom macOS theme changing code. Thanks for spotting that!

[F0bes]

Works fine on macOS.