Jan Grulich

gtk

Qt theming in Fedora Workstation

- jgrulich 14 Comments

We have been working on and using custom Qt theming in Fedora Workstation for many years now. By custom Qt theming, I’m talking about the QGnomePlatform and Adwaita-qt projects. If you haven’t heard of them, you can read my recent blog post explaining what they are. While these projects are in some ways better than what Qt upstream has to offer, there were also drawbacks/issues and that’s why I decided to make a final decision and discontinue both projects. The issues are explained in the aforementioned blog post, but one of the main drawbacks is that we are in this development alone and not working directly in the upstream makes it less attractive for other contributors. It’s also not used by default anywhere other than Fedora, so it’s not properly tested by other developers working on Qt applications using different distributions. These reasons led me to submit a Fedora 39 feature to remove our custom Qt theming in Fedora Workstation in favor of Qt’s defaults. The only problem is that if we just go with Qt’s default, we would go backwards a bit. This is because upstream Qt does not provide any decent client-side window decorations (problem #1), and the QGtkTheme in Qt5 (QGnomePlatform equivalent) is a bit behind its Qt6 version with many improvements and integration goodies (problem #2) recently made by Axel Spoerl of the Qt Group, whom I met during this year’s KDE Akademy.

Solution to problem #1

QGnomePlatform used to be our solution to this problem, as QGnomePlatform implemented it’s own version of the QWaylandAbstractDecoration plugin. This was a GTK 3-like decoration plugin that used Adwaita-qt for button rendering and QGnomePlatform bits (e.g. GSettings configuration) to get the titlebar layout. Since we are going to remove QGnomePlatform, we needed an alternative. So I started working on the QAdwaitaDecorations project. This is supposed to be a an intermediate step as I would like to have a proper GNOME/Gtk decorations directly in Qt upstream, but since I was in a hurry to get everything done in time for Fedora 39, we have this for now. QAdwaitaDecorations plugin is based on the decorations we have in QGnomePlatform, but there is no dependency on GTK or Glib (e.g. GSettings) or on Adwaita-qt. We use xdg-desktop-portal to get the titlebar layout and do our own drawing instead. This decoration plugin should also have now a GTK 4-like style so the buttons and colors of the decorations are different.

Below is a screenshot of Wireshark (Qt6) using QAdwaitaDecorations plugin + QGtkTheme + Fusion:

Solution to problem #2

Since Qt5 is no longer actively developed, the only possible solution is to backport all QGtkTheme improvements from Qt6, so I did that + modified some of those changes to avoid breaking binary compatibility. This results in about ~15 related backports to Qt5 so far, and it seems to work pretty well. I also made sure that Fedora 38 and older will still use QGnomePlatform by default, so we don’t change the behavior for existing users. Also, a small change to our QtWayland package was needed to make it use the new decoration plugin by default.

Future plans

As mentioned, I would really like to have everything directly in Qt upstream (talking about QAdwaitaDecorations). That way we get other contributions and thus fixes/improvements for free and a lot more users. Another thing is that QGnomePlatform supports things that are not yet supported/implemented in QGtkTheme, like support for xdg-desktop-portal instead of just relying on GSettings. Not to mention that GTK 4 has been around for a while, and both QGnomePlatform and QGtkTheme are still GTK 3 based. I will definitely try to make some of these things happen for Fedora 40, but knowing myself, it’s better not to make any promises, as things usually don’t go according to plan.

HighContrast variants for Adwaita-qt

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In the past we used to have a completely different project to cover HighContrast variants of GTK Adwaita theme. This was all implemented as Highcontrast-qt, a project nobody has touched for 6 years. You can imagine how it looks like these days when you compare it to what we have now. I think even GTK variant of HighContrast was a completely separate theme back then, while today days it’s just Adwaita with a different set of colors.

Since GTK made the new HighContrast theme with just few modifications to the original Adwaita theme, I decided to use same approach and have Adwaita-qt to provide all four variants as well (Adwaita, Adwaita-dark, HighContrast and HighContrastInverse). While this looks like a simple thing to do, as you just need to add additional color palette, it was a pain to do it in Adwaita-qt. The reason is that Adwaita-qt is full of hardcoded color definitions, where all of them were randomly taken from GTK Adwaita stylesheets. Everytime something changed in GTK Adwaita, we would have to manually pick the change and replace the changed color value on our side. This was not really sustainable, especially when I wanted to have four different variants.

To improve this situation and make my life easier, I decided to bundle GTK stylesheets for Adwaita theme, have it processed and write a simple parser to make everything automated. And I did exactly that. I included the stylesheets and wrote some definitions myself so I can let them processed with sassc (GTK uses SASS for theming) and have all my definitions substituted for simple parsing. I no longer have to pick all base colors manually, all of them are being parsed for all four variants and same goes for basic styling of Buttons, CheckBoxes and Radio Buttons, where each of them have all kind of possible states (active, hovered, checked etc.) and use not only a simple color, but also gradients. You can imagine how hard it was to hardcode all values for each state. The parser I wrote is really basic and simple with use of regular expressions as the code I’m trying to process is not that complex.

The code I try to process is either a simple definition:

@define-color base_color #ffffff;

Or widget definition in this form:

button:checked { color: #2e3436; border-color: #cdc7c2; background-image: image(#dad6d2); box-shadow: none; }

The result is:

Adwaita-qt: Adwaita variant
Adwaita-qt: HighContrast variant

I think this is a big step forward for Adwaita-qt and it will allow me to more quicky respond on changes happening in GTK Adwaita/HighContrast theme. I can also imagine this being extended in the future to support some additional variants, like modified Adwaita theme you can find in Ubuntu (at least if the stylesheet is similar enough). As mentioned, Adwaita-qt now supports four variants and they should be on par with GTK 4.4, at least when it comes to colors and style for most used widgets, because there are still places in Adwaita-qt which need some extra work. Anyway, this all is now released as Adwaita-qt 1.4.0 and I will be updating Flathub and Fedora builds to it soon.

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