Qt backend: animation of window properties doesn't work

[task-jp]

Bug Description

I'm attempting to animate the background color of a Window with animation-tick(), but it doesn't work as expected when using the Qt backend:

Expected Behavior
The background color should continuously animate without requiring user interaction, even when using the Qt backend.
Actual Behavior
The background color only updates on user interaction (window, mouse, or keyboard events) when using the Qt backend.

When I set SLINT_NO_QT=1 or use slintpad, it behaves correctly and continuously animates the background color.

Reproducible Code (if applicable)

export component App inherits Window {
    background: red.mix(blue, (sin(animation-tick() / 1s * 360deg) + 1) / 2);
}

Environment Details

• Slint Version: 1.8.0, 1.9.1
• Platform/OS: Kubuntu 24.04/X11
• Programming Language:
• Backend/Renderer: Qt

Product Impact

No response

[ogoffart]

The Qt backend queries the background in
update_window_properties(&self, properties: WindowProperties<'_>>

But for some reason, this doesn't seem to be called when the properties are dirty.
Another way to reproduce:

export component Example inherits Window {
        title: "Hi " + (animation-tick()/1s);
}

The title change with the winit backend but not with the Qt backend

[ogoffart]

The problem is that the animation-tick() property doesn't change until update_timers_and_animations is called. And this is not call unless something else happens for Qt.