pub fn handle_single_border(&mut self, border_width: i32) {
        if self.single_window_border {
            return;
        }

        for tag in self.tags.normal() {
            let mut windows_on_tag: Vec<&mut Window<H>> = self
                .windows
                .iter_mut()
                .filter(|w| w.tag.unwrap_or(0) == tag.id && w.r#type == WindowType::Normal)
                .collect();

            let wsid = self
                .workspaces
                .iter()
                .find(|ws| ws.has_tag(&tag.id))
                .map(|w| w.id);
            let layout = self.layout_manager.layout(wsid.unwrap_or(1), tag.id);

            // TODO: hardcoded layout name.
            if layout.is_monocle() {
                windows_on_tag.iter_mut().for_each(|w| w.border = 0);
                continue;
            }

            if windows_on_tag.len() == 1 {
                if let Some(w) = windows_on_tag.first_mut() {
                    w.border = 0;
                }
                continue;
            }

            windows_on_tag
                .iter_mut()
                .for_each(|w| w.border = border_width);
        }
    }
Removes border if there is a single visible window.
Only will run if `single_window_border` is set to `false` in the configuration file.
leftwm-core/src/state.rs