memory leak of Plasmashell when displaying a Timer value on a window with embedded icon on X11 with Qt backend

[nezmaka]

Bug Description

Hello,

I've noticed an apparent memory leak.
The system it happens on is running Kubuntu 24.04 and X11. It occurs with the qt backend (Qt5). Setting SLINT_BACKEND to winit-skia prevents it from happening.

The code that produces the memory leak is as follows:

• a slint Timer component is running
• the timer's value is displayed in a Text component
• an embedded image is used as an icon for the window.

The memory leak takes the form of KDE Plasma's plasmashell process' RAM usage monotonically increasing. This leaks occurs only when the three conditions above hold true. An image embedded in an Image {} element does not cause a leak. If the timer is stopped, the increase also stops. After the app is shut down, plasmashell's RAM usage does not go down.

I'm providing a minimal code to reproduce. For it to run, you'd have to include a github-logo.png in the ui directory to serve as an icon.

app-window.slint

export component AppWindow inherits Window {
  property <int> global-time-value: 0;
  icon: @image-url("github-logo.png"); // <- commenting out prevents leak

  global_timer := Timer {
      interval: 10ms;
      running: false;
      triggered() => {
          global-time-value += 1;
      }
  }
  VerticalBox {
      Text {
          text: root.global_time_value; // <- replacing with static text prevents leak
          horizontal-alignment: right;
      }

      Image { 
  source: @image-url("github-logo.png");  // <- does not cause a memory leak
      }

Button {
          text: "Start";
          clicked => {
              global_timer.start();
          }
      }
Button {
          text: "Stop";
          clicked => {
              global_timer.stop();
          }
      }
  }
}

main.rs

use std::error::Error;

slint::include_modules!();

fn main() -> Result<(), Box<dyn Error>> {
  let ui = AppWindow::new()?;

  ui.run()?;

  Ok(())
}

Reproducible Code (if applicable)

Environment Details

• Slint Version: 1.12.1
• Platform/OS: Kubuntu 24.04, X11
• Programming Language: Rust
• Backend/Renderer: Qt5

Product Impact

No response

[ogoffart]

Looks like a bug in plasmashell.

Here is what I think happens:

• Changing the Text causes the layout to re-evaluate its constraint
• Since the minimum size might have changed, we call the update_window_properties() in the platform
• This call QWidget::setWindowIcon even if the icon didn't change
• There is a leak in Plasma apparently (or too bigger caches?)

We could try to workaround the bug by not changing the icon if it hasn't changed. The winit platform already does that.