Function display_scale

native/linux/src/lib.rs:196–206

Read the current display's DPI scale factor. Computed from physical screen dimensions (pixels / mm). Snapped to the nearest 0.25 and clamped to [1.0, 4.0] — EDID often lies about millimetres so we want stable integer-ish steps. A real Wayland-style desktop environment with explicit scale settings is out of scope here; this matches what most X11 users actually run (fractional scaling via Xft.dpi is

(display: *mut x11::xlib::Display, screen: i32)

    /// users actually run (fractional scaling via Xft.dpi is the
    /// only thing GTK/Qt honour by default).
    pub fn display_scale(display: *mut x11::xlib::Display, screen: i32) -> f64 {
        unsafe {
            let pixels = x11::xlib::XDisplayWidth(display, screen) as f64;
            let mm = x11::xlib::XDisplayWidthMM(display, screen) as f64;
            if mm <= 0.0 || pixels <= 0.0 { return 1.0; }
            let dpi = pixels / (mm / 25.4);
            // 96 DPI = scale 1.0. Snap to 0.25 steps.
            let raw = (dpi / 96.0).max(1.0).min(4.0);
            (raw * 4.0).round() / 4.0
        }
    }

    pub fn display() -> *mut x11::xlib::Display { unsafe { DISPLAY } }
    pub fn window() -> x11::xlib::Window { unsafe { X11_WINDOW } }