Egui inner shadow

egui_inner_shadow.rs

#[extension_trait]
pub impl UiExt for Ui {
  fn inner_shadow_top(&mut self, mut avail_rect: Rect, opacity: f32) {
    let painter = self.painter();
    avail_rect.set_height(1.0);
    const STEPS: usize = 8;
    for i in 0..STEPS {
      let alpha = 1.0 - (i as f32 / STEPS as f32);
      let shift = avail_rect.height() * i as f32;
      let rect = avail_rect.translate((0.0, shift).into());
      painter.rect_filled(rect, 0.0, Color32::from_black_alpha((alpha * alpha * 70.0 * opacity).floor() as u8));
    }
  }

  fn inner_shadow_bottom(&mut self, avail_rect: Rect, opacity: f32) {
    let painter = self.painter();
    let mut avail_rect = avail_rect.translate((0.0, avail_rect.height() - 1.0).into());
    avail_rect.set_height(1.0);
    const STEPS: usize = 8;
    for i in 0..STEPS {
      let alpha = 1.0 - (i as f32 / STEPS as f32);
      let shift = -avail_rect.height() * i as f32;
      let rect = avail_rect.translate((0.0, shift).into());
      painter.rect_filled(rect, 0.0, Color32::from_black_alpha((alpha * alpha * 80.0 * opacity).floor() as u8));
    }
  }
}

#[extension_trait]
pub impl<T> ScrollAreaOutputExt for ScrollAreaOutput<T> {
  fn inner_shadow(self, ui: &mut Ui) -> Self {
    const RAMP: f32 = 12.0;
    let mut shadow_rect = self.inner_rect;
    shadow_rect.set_width(shadow_rect.width() + ui.spacing().scroll_bar_inner_margin);

    let opacity = self.state.offset.y.min(RAMP).max(0.0) / RAMP;
    ui.inner_shadow_top(shadow_rect, opacity);

    let opacity = (self.content_size.y - self.state.offset.y - self.inner_rect.height()).min(RAMP).max(0.0) / RAMP;
    ui.inner_shadow_bottom(shadow_rect, opacity);
    self
  }
}

example_usage.rs

egui::ScrollArea::vertical()
  .auto_shrink([false, false])
  .show(ui, |ui| {
    // Contents here
  })
  .inner_shadow(ui); // <--------------- Shadows added here