[BUG] The css_classes() in the GtkWidget builder is problematic

[melix99]

Bug description

When creating a widget using a builder and using its function css_classes to set custom CSS classes, what it does is calling its internal setter set_css_classes which clears any existing css classes and then it adds the one defined by the user. The problem with this is that it also removes css classes that are set by GTK internally, so this often result to undesired behavior. This is an example:

let button = gtk::Button::builder()
    .icon_name("media-playback-start-symbolic")
    .halign(gtk::Align::Center)
    .valign(gtk::Align::Center)
    .css_classes(vec!["flat".into()])
    .build();

In this example, GTK normally adds the css class "image-button" to style the button correctly (in this case it tweaks internal spacing), but with the code above, because of the css_classes, it removes that class and only set the "flat" CSS class. In this specific case it causes the widget to not have a perfectly squared allocation. I guess this can create bigger problems with other combinations.

I think that css_classes() should probably be changed to only add CSS classes to avoid confusion.

[bilelmoussaoui]

Should it? that is the behaviour of such "property" in general, so I don't feel like we should do anything about it.

[melix99]

Should it? that is the behaviour of such "property" in general, so I don't feel like we should do anything about it.

Well, honestly it was quite annoying to debug since I expected that function to behave like the XML "<style>" UI definition, where it does not behave like that.

[bilelmoussaoui]

That would be an issue to report to gtk itself I guess, to either document the behaviour clearly or to make it keep the default CSS classes somehow

[bilelmoussaoui]

An alternative would be removing the setter from the builder pattern, but we don't have any way to configure the builder pattern yet.

[bilelmoussaoui]

See gtk-rs/gir#1302