Fixes: preserve taskbar monitor selection + widget vanishing when start menu opens

src/native_interop.rs

@@ -1,5 +1,8 @@
 use windows::core::PCWSTR;
 use windows::Win32::Foundation::{BOOL, HWND, LPARAM, RECT};
+use windows::Win32::Graphics::Gdi::{
+    GetMonitorInfoW, MonitorFromWindow, MONITORINFOEXW, MONITOR_DEFAULTTONEAREST,
+};
 use windows::Win32::UI::Accessibility::{SetWinEventHook, UnhookWinEvent, HWINEVENTHOOK};
 use windows::Win32::UI::Shell::{SHAppBarMessage, ABM_GETTASKBARPOS, APPBARDATA};
 use windows::Win32::UI::WindowsAndMessaging::*;
@@ -24,10 +27,33 @@ pub const WM_APP: u32 = 0x8000;
 pub const WM_APP_USAGE_UPDATED: u32 = WM_APP + 1;
 pub const WM_APP_TRAY: u32 = WM_APP + 3;
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Debug)]
 pub struct TaskbarWindow {
     pub hwnd: HWND,
     pub rect: RECT,
+    /// Stable monitor identity (e.g. "\\\\.\\DISPLAY1"). Used to keep the widget
+    /// anchored to the same physical monitor across topology changes, where the
+    /// geometric ordering (and therefore the index) of taskbars can shift.
+    pub device: String,
+}
+
+/// Stable identifier of the monitor a window currently lives on.
+pub fn monitor_device_name(hwnd: HWND) -> String {
+    unsafe {
+        let monitor = MonitorFromWindow(hwnd, MONITOR_DEFAULTTONEAREST);
+        let mut info = MONITORINFOEXW::default();
+        info.monitorInfo.cbSize = std::mem::size_of::<MONITORINFOEXW>() as u32;
+        if GetMonitorInfoW(monitor, &mut info as *mut _ as *mut _).as_bool() {
+            let len = info
+                .szDevice
+                .iter()
+                .position(|&c| c == 0)
+                .unwrap_or(info.szDevice.len());
+            String::from_utf16_lossy(&info.szDevice[..len])
+        } else {
+            String::new()
+        }
+    }
 }
 
 pub fn find_taskbars() -> Vec<TaskbarWindow> {
@@ -39,7 +65,8 @@ pub fn find_taskbars() -> Vec<TaskbarWindow> {
             let class_name = String::from_utf16_lossy(&class_name[..len as usize]);
             if class_name == "Shell_TrayWnd" || class_name == "Shell_SecondaryTrayWnd" {
                 if let Some(rect) = get_taskbar_rect(hwnd).or_else(|| get_window_rect_safe(hwnd)) {
-                    taskbars.push(TaskbarWindow { hwnd, rect });
+                    let device = monitor_device_name(hwnd);
+                    taskbars.push(TaskbarWindow { hwnd, rect, device });
                 }
             }
         }
@@ -169,6 +196,10 @@ pub fn get_window_thread_id(hwnd: HWND) -> u32 {
     unsafe { GetWindowThreadProcessId(hwnd, None) }
 }
 
+pub fn window_exists(hwnd: HWND) -> bool {
+    unsafe { IsWindow(hwnd).as_bool() }
+}
+
 /// Unhook a WinEvent hook
 pub fn unhook_win_event(hook: HWINEVENTHOOK) {
     unsafe {

src/window.rs

@@ -84,6 +84,7 @@ struct AppState {
     last_update_check_unix: Option<u64>,
 
     taskbar_index: usize,
+    taskbar_device: Option<String>,
     tray_offset: i32,
     dragging: bool,
     drag_start_mouse_x: i32,
@@ -140,6 +141,8 @@ const TRAY_ICON_UPDATE_REPOSITION_SUPPRESS_MS: u64 = 750;
 /// recreates the taskbar and wipes our tray-icon registration).
 const TASKBAR_WATCH_INTERVAL_SECS: u64 = 2;
 
+const TASKBAR_MISSING_TICKS: u32 = 2;
+
 static SUPPRESS_TRAY_REPOSITION_UNTIL: Mutex<Option<Instant>> = Mutex::new(None);
 
 /// Current system DPI (96 = 100% scaling, 144 = 150%, 192 = 200%, etc.)
@@ -232,21 +235,35 @@ fn relaunch_self() {
 /// dedicated thread (independent of the dead message loop) polls the taskbar
 /// handle and, when it changes, relaunches the widget as a fresh process.
 fn spawn_taskbar_watchdog() {
-    std::thread::spawn(move || loop {
-        std::thread::sleep(Duration::from_secs(TASKBAR_WATCH_INTERVAL_SECS));
-        let stored = {
-            let state = lock_state();
-            state.as_ref().and_then(|s| s.taskbar_hwnd)
-        };
-        // Only relevant once we have embedded into a taskbar at least once.
-        let Some(old) = stored else {
-            continue;
-        };
-        let taskbars = native_interop::find_taskbars();
-        if !taskbars.is_empty() && !taskbars.iter().any(|taskbar| taskbar.hwnd == old) {
+    std::thread::spawn(move || {
+        let mut missing_streak = 0u32;
+        loop {
+            std::thread::sleep(Duration::from_secs(TASKBAR_WATCH_INTERVAL_SECS));
+            let stored = {
+                let state = lock_state();
+                state.as_ref().and_then(|s| s.taskbar_hwnd)
+            };
+            let Some(old) = stored else {
+                missing_streak = 0;
+                continue;
+            };
+            if native_interop::window_exists(old) {
+                missing_streak = 0;
+                continue;
+            }
+            let taskbars = native_interop::find_taskbars();
+            if taskbars.is_empty() || taskbars.iter().any(|taskbar| taskbar.hwnd == old) {
+                missing_streak = 0;
+                continue;
+            }
+            missing_streak += 1;
+            if missing_streak < TASKBAR_MISSING_TICKS {
+                continue;
+            }
+            missing_streak = 0;
             let new = taskbars[0].hwnd;
             diagnose::log(format!(
-                "watchdog: taskbar changed old={:?} new={:?} -> relaunching",
+                "watchdog: taskbar destroyed old={:?} new={:?} -> relaunching",
                 old.0, new.0
             ));
             relaunch_self();
@@ -302,6 +319,8 @@ struct SettingsFile {
     tray_offset: i32,
     #[serde(default)]
     taskbar_index: usize,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    taskbar_device: Option<String>,
     #[serde(default = "default_poll_interval")]
     poll_interval_ms: u32,
     #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -323,6 +342,7 @@ impl Default for SettingsFile {
         Self {
             tray_offset: 0,
             taskbar_index: 0,
+            taskbar_device: None,
             poll_interval_ms: default_poll_interval(),
             language: None,
             last_update_check_unix: None,
@@ -382,6 +402,7 @@ fn save_state_settings() {
         save_settings(&SettingsFile {
             tray_offset: s.tray_offset,
             taskbar_index: s.taskbar_index,
+            taskbar_device: s.taskbar_device.clone(),
             poll_interval_ms: s.poll_interval_ms,
             language: s
                 .language_override
@@ -494,17 +515,28 @@ fn toggle_widget_visibility(hwnd: HWND) {
     }
 }
 
-fn attach_to_taskbar(hwnd: HWND, requested_index: usize) -> bool {
+fn attach_to_taskbar(
+    hwnd: HWND,
+    requested_index: usize,
+    requested_device: Option<&str>,
+) -> bool {
     let taskbars = native_interop::find_taskbars();
     if taskbars.is_empty() {
         diagnose::log("taskbar not found; using fallback popup window");
         return false;
     }
 
-    let index = requested_index.min(taskbars.len().saturating_sub(1));
-    let taskbar = taskbars[index];
+    // Prefer the taskbar on the same physical monitor we were last anchored to.
+    // The geometric index is unstable across monitor connect/disconnect/reorder,
+    // so matching by device name keeps the widget on the user's chosen monitor.
+    let index = requested_device
+        .filter(|d| !d.is_empty())
+        .and_then(|device| taskbars.iter().position(|t| t.device == device))
+        .unwrap_or_else(|| requested_index.min(taskbars.len().saturating_sub(1)));
+    let taskbar = taskbars[index].clone();
     diagnose::log(format!(
-        "taskbar selected index={index} count={} hwnd={:?} rect=({}, {}, {}, {})",
+        "taskbar selected index={index} device={} count={} hwnd={:?} rect=({}, {}, {}, {})",
+        taskbar.device,
         taskbars.len(),
         taskbar.hwnd,
         taskbar.rect.left,
@@ -546,6 +578,7 @@ fn attach_to_taskbar(hwnd: HWND, requested_index: usize) -> bool {
         s.tray_notify_hwnd = tray_notify;
         s.win_event_hook = hook;
         s.taskbar_index = index;
+        s.taskbar_device = Some(taskbar.device.clone());
         s.embedded = true;
     }
     true
@@ -1321,6 +1354,7 @@ pub fn run() {
                 update_status: UpdateStatus::Idle,
                 last_update_check_unix: settings.last_update_check_unix,
                 taskbar_index: settings.taskbar_index,
+                taskbar_device: settings.taskbar_device.clone(),
                 tray_offset: settings.tray_offset,
                 dragging: false,
                 drag_start_mouse_x: 0,
@@ -1331,7 +1365,7 @@ pub fn run() {
         }
 
         // Try to embed in taskbar
-        if attach_to_taskbar(hwnd, settings.taskbar_index) {
+        if attach_to_taskbar(hwnd, settings.taskbar_index, settings.taskbar_device.as_deref()) {
             embedded = true;
         }
 
@@ -2487,7 +2521,7 @@ unsafe extern "system" fn wnd_proc(
                                 s.tray_offset = new_offset;
                             }
                         }
-                        if attach_to_taskbar(hwnd, target_index) {
+                        if attach_to_taskbar(hwnd, target_index, Some(&target_taskbar.device)) {
                             position_at_taskbar();
                             render_layered();
                         }