Resolve DRM cursor density from the complete display mapping

src/server/drm_capturer.rs

@@ -994,19 +994,35 @@ pub fn drm_cursor_snapshot<T>(
     let monitor = if hidden {
         None
     } else {
-        display_info_of(display).and_then(|display| {
-            // Display discovery runs outside the cursor service; missing metadata means unknown DPI.
-            let wayland = scrap::wayland::display::get_cached_displays()?;
-            let index = identity_matches(&[display], &wayland.displays)
-                .into_iter()
-                .next()
-                .flatten()?;
-            wayland.displays.get(index).cloned()
+        // Display discovery runs outside the cursor service; missing metadata means unknown DPI.
+        scrap::wayland::display::get_cached_displays().and_then(|wayland| {
+            cursor_monitor(
+                display.max(0) as usize,
+                &DRM_STATE.lock().unwrap(),
+                &wayland.displays,
+            )
         })
     };
     Some((value, monitor))
 }
 
+#[cfg(feature = "flutter")]
+fn cursor_monitor(
+    display: usize,
+    state: &ProbeState,
+    monitors: &[base::platform::linux::WaylandDisplayInfo],
+) -> Option<base::platform::linux::WaylandDisplayInfo> {
+    let ProbeState::Available(_, displays) = state else {
+        return None;
+    };
+    // Reserve every connector's name match before guessing this cursor's density.
+    let index = identity_matches(displays, monitors)
+        .get(display)
+        .copied()
+        .flatten()?;
+    monitors.get(index).cloned()
+}
+
 enum ProbeState {
     Unknown,
     Unavailable(Instant),
@@ -2303,6 +2319,36 @@ mod drm_capturer_tests {
         assert!(m2[0].is_none() && m2[1].is_none());
     }
 
+    #[cfg(feature = "flutter")]
+    #[test]
+    fn cursor_monitor_reserves_other_displays_before_guessing_density() {
+        let state = ProbeState::Available(
+            Instant::now(),
+            vec![
+                drm_display("DSI-1", 1920, 1080),
+                drm_display("HDMI-A-1", 1920, 1080),
+            ],
+        );
+        for other_width in [2560, 1920] {
+            let mut monitors = vec![
+                wl_display("HDMI-1", 0, 0, 1920, 1080),
+                wl_display("Unknown-9", 960, 0, other_width, 1080),
+            ];
+            monitors[0].logical_size = Some((960, 540));
+            let selected = cursor_monitor(0, &state, &monitors);
+            if other_width == 2560 {
+                assert!(selected.is_none(), "DSI must not inherit HDMI's 2x density");
+            } else {
+                let selected = selected.unwrap();
+                assert_eq!(selected.name, "Unknown-9");
+                assert_eq!(selected.logical_size, Some((1920, 1080)));
+            }
+            let hdmi = cursor_monitor(1, &state, &monitors).unwrap();
+            assert_eq!(hdmi.name, "HDMI-1");
+            assert_eq!(hdmi.logical_size, Some((960, 540)));
+        }
+    }
+
     #[test]
     fn outputs_are_matched_by_name_across_the_drm_naming_difference() {
         let drm = [drm_display("HDMI-A-1", 1920, 1080), drm_display("DP-1", 2560, 1440)];