feat: raise covered bubbles the moment another app's window appears

A WinEvent hook on window shows and z-order changes re-raises a bubble that another process's always-on-top window covers, instead of waiting for the 1.5 s check, which stays as a backstop. Raises are capped at 20 a second so an app that also fights for the top cannot keep both busy.

Cargo.toml

@@ -56,6 +56,7 @@ windows = { version = "0.62", features = [
     "Win32_Foundation",
     "Win32_System_Registry",
     "Win32_System_Threading",
+    "Win32_UI_Accessibility",
     "Win32_UI_WindowsAndMessaging",
     "Win32_UI_Shell",
     "Win32_Graphics_Dwm",

README.md

@@ -46,9 +46,9 @@ self-updater are all written from scratch against the same public APIs
 - Auto-hide when a fullscreen app is in the foreground (games, video,
   presentations) — reappears when you leave fullscreen
 - Bubbles stay above other always-on-top windows, such as chat
-  notifications: on Windows a covered bubble is raised again within about
-  1.5 s, and on every platform shown bubbles are put back on top every
-  minute, so nothing can keep them covered or hidden
+  notifications: on Windows a bubble goes back on top the moment another
+  app shows such a window over it, and on every platform shown bubbles are
+  put back on top every minute, so nothing can keep them covered or hidden
 
 ## Who this is for
 

src/bubble.rs

@@ -15,9 +15,7 @@
 use std::collections::HashMap;
 use std::sync::atomic::{AtomicU64, Ordering};
 use std::sync::{Arc, Mutex, PoisonError};
-use std::time::Duration;
-#[cfg(not(windows))]
-use std::time::Instant;
+use std::time::{Duration, Instant};
 
 use tauri::{
     AppHandle, Manager, Monitor, PhysicalPosition, PhysicalSize, WebviewUrl, WebviewWindow,
@@ -416,6 +414,7 @@ fn prepare_native(
         if click_through {
             set_ignore_cursor(&window, true);
         }
+        watch_z_order(&hook_app);
         let hooked = platform::windows::install_bubble_hook(&window, move |event| {
             on_hook_event(&hook_app, provider, event);
         });
@@ -536,24 +535,69 @@ pub fn bring_back(app: &AppHandle) {
 #[cfg(windows)]
 pub fn keep_on_top(app: &AppHandle) {
     let handle = app.clone();
-    let queued = app.run_on_main_thread(move || {
-        let shows = runtime_of(&handle).is_some_and(|r| r.with_state(|s| s.visibility.shows()));
-        if !shows {
-            return;
-        }
-        for (_, window) in windows(&handle) {
-            match platform::windows::covered_by_other_topmost(&window) {
-                Ok(true) => raise(&window),
-                Ok(false) => {}
-                Err(err) => log::warn!("{} on-top check failed: {err}", window.label()),
-            }
-        }
-    });
+    let queued = app.run_on_main_thread(move || raise_covered(&handle));
     if let Err(err) = queued {
         log::warn!("bubble on-top check could not reach the main thread: {err}");
     }
 }
 
+/// Most raises in [`RAISE_WINDOW`] before raising pauses until it ends, so
+/// another app that also fights for the top cannot keep both busy; the
+/// fullscreen check carries on at its own pace.
+#[cfg(windows)]
+const MAX_RAISES: u32 = 20;
+#[cfg(windows)]
+const RAISE_WINDOW: Duration = Duration::from_secs(1);
+
+/// Body of [`keep_on_top`]; runs on the main thread.
+#[cfg(windows)]
+fn raise_covered(app: &AppHandle) {
+    static RAISES: Mutex<Option<(Instant, u32)>> = Mutex::new(None);
+    let shows = runtime_of(app).is_some_and(|r| r.with_state(|s| s.visibility.shows()));
+    if !shows {
+        return;
+    }
+    for (_, window) in windows(app) {
+        match platform::windows::covered_by_other_topmost(&window) {
+            Ok(true) => {}
+            Ok(false) => continue,
+            Err(err) => {
+                log::warn!("{} on-top check failed: {err}", window.label());
+                continue;
+            }
+        }
+        let allowed = {
+            let mut raises = RAISES.lock().unwrap_or_else(PoisonError::into_inner);
+            let now = Instant::now();
+            let (start, count) = raises.get_or_insert((now, 0));
+            if now.duration_since(*start) >= RAISE_WINDOW {
+                (*start, *count) = (now, 0);
+            }
+            *count += 1;
+            if *count == MAX_RAISES + 1 {
+                log::warn!("bubble raises paused: another window keeps taking the top");
+            }
+            *count <= MAX_RAISES
+        };
+        if allowed {
+            raise(&window);
+        }
+    }
+}
+
+/// Raise covered bubbles the moment another app shows a window or changes
+/// the z-order. Installed once, on the main thread.
+#[cfg(windows)]
+fn watch_z_order(app: &AppHandle) {
+    static WATCHING: std::sync::Once = std::sync::Once::new();
+    WATCHING.call_once(|| {
+        let handle = app.clone();
+        if let Err(err) = platform::windows::watch_z_order(move || raise_covered(&handle)) {
+            log::warn!("z-order watch unavailable, relying on the periodic check: {err}");
+        }
+    });
+}
+
 fn raise(window: &WebviewWindow) {
     #[cfg(windows)]
     let result = platform::windows::raise_topmost(window).map_err(|e| e.to_string());

src/fullscreen.rs

@@ -6,7 +6,8 @@
 //! The same thread brings the bubbles back every minute while they should
 //! show, in case another window covered or hid them. On Windows it also
 //! re-raises, on every check, a bubble that another app's always-on-top
-//! window (a chat toast, say) has just covered.
+//! window (a chat toast, say) covers, as a backstop for the z-order watch
+//! in `bubble.rs`.
 
 use std::io;
 use std::sync::Arc;

src/platform/windows.rs

@@ -1,5 +1,6 @@
 //! Windows-only helpers. This is the only module that talks to Win32 APIs.
 
+use std::cell::RefCell;
 use std::io;
 use std::path::Path;
 use std::time::Duration;
@@ -20,6 +21,7 @@ use windows::Win32::Graphics::Gdi::{
     GetMonitorInfoW, MonitorFromWindow, MONITORINFO, MONITOR_DEFAULTTONEAREST,
 };
 use windows::Win32::System::Threading::{OpenProcess, WaitForSingleObject, PROCESS_SYNCHRONIZE};
+use windows::Win32::UI::Accessibility::{SetWinEventHook, HWINEVENTHOOK};
 use windows::Win32::UI::Shell::{
     DefSubclassProc, RemoveWindowSubclass, SHAppBarMessage,
     SetCurrentProcessExplicitAppUserModelID, SetWindowSubclass, ABE_BOTTOM, ABE_LEFT, ABE_RIGHT,
@@ -28,11 +30,13 @@ use windows::Win32::UI::Shell::{
 use windows::Win32::UI::WindowsAndMessaging::{
     GetClassNameW, GetForegroundWindow, GetWindow, GetWindowLongPtrW, GetWindowRect,
     GetWindowThreadProcessId, IsIconic, IsWindowVisible, MessageBoxW, SetForegroundWindow,
-    SetWindowLongPtrW, SetWindowPos, ShowWindow, GWL_EXSTYLE, GWL_STYLE, GW_HWNDPREV, HWND_TOPMOST,
-    MA_NOACTIVATE, MB_ICONERROR, MB_OK, MB_SETFOREGROUND, SWP_FRAMECHANGED, SWP_NOACTIVATE,
-    SWP_NOMOVE, SWP_NOSIZE, SWP_NOZORDER, SW_HIDE, SW_SHOWNOACTIVATE, WM_DISPLAYCHANGE,
-    WM_EXITSIZEMOVE, WM_MOUSEACTIVATE, WM_NCDESTROY, WM_SETTINGCHANGE, WS_EX_LAYERED,
-    WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW, WS_EX_TOPMOST, WS_EX_TRANSPARENT,
+    SetWindowLongPtrW, SetWindowPos, ShowWindow, CHILDID_SELF, EVENT_OBJECT_REORDER,
+    EVENT_OBJECT_SHOW, GWL_EXSTYLE, GWL_STYLE, GW_HWNDPREV, HWND_TOPMOST, MA_NOACTIVATE,
+    MB_ICONERROR, MB_OK, MB_SETFOREGROUND, OBJID_WINDOW, SWP_FRAMECHANGED, SWP_NOACTIVATE,
+    SWP_NOMOVE, SWP_NOSIZE, SWP_NOZORDER, SW_HIDE, SW_SHOWNOACTIVATE, WINEVENT_OUTOFCONTEXT,
+    WINEVENT_SKIPOWNPROCESS, WM_DISPLAYCHANGE, WM_EXITSIZEMOVE, WM_MOUSEACTIVATE, WM_NCDESTROY,
+    WM_SETTINGCHANGE, WS_EX_LAYERED, WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW, WS_EX_TOPMOST,
+    WS_EX_TRANSPARENT,
 };
 
 use crate::core::geom::{self, Edge, Rect};
@@ -372,6 +376,67 @@ fn covers(other: HWND, ours: &Rect, own_pid: u32) -> bool {
     visible_window_bounds(other).is_some_and(|(theirs, _)| theirs.intersects(ours))
 }
 
+thread_local! {
+    /// The callback of [`watch_z_order`], on the thread that installed it.
+    static Z_ORDER_CALLBACK: RefCell<Option<Box<dyn Fn()>>> = const { RefCell::new(None) };
+}
+
+/// Call `callback` as soon as a window of another process is shown or the
+/// z-order changes, so a bubble can go back on top of a new always-on-top
+/// window (a chat toast, say) at once instead of at the next poll. The
+/// events arrive through this thread's message loop, so install it on the
+/// main thread; the hook lives as long as the process.
+pub fn watch_z_order(callback: impl Fn() + 'static) -> io::Result<()> {
+    Z_ORDER_CALLBACK.with(|slot| *slot.borrow_mut() = Some(Box::new(callback)));
+    // SAFETY: an out-of-context hook needs no module handle; the procedure
+    // is a plain function that lives for the whole process.
+    let hook = unsafe {
+        SetWinEventHook(
+            EVENT_OBJECT_SHOW,
+            EVENT_OBJECT_REORDER,
+            None,
+            Some(z_order_event_proc),
+            0,
+            0,
+            WINEVENT_OUTOFCONTEXT | WINEVENT_SKIPOWNPROCESS,
+        )
+    };
+    if hook.is_invalid() {
+        Z_ORDER_CALLBACK.with(|slot| slot.borrow_mut().take());
+        return Err(io::Error::other("SetWinEventHook failed"));
+    }
+    Ok(())
+}
+
+unsafe extern "system" fn z_order_event_proc(
+    _hook: HWINEVENTHOOK,
+    event: u32,
+    _hwnd: HWND,
+    id_object: i32,
+    id_child: i32,
+    _thread: u32,
+    _time: u32,
+) {
+    // Only whole windows count: a shown caret or cursor is an object too,
+    // and a hidden window cannot cover anything.
+    let relevant = match event {
+        EVENT_OBJECT_SHOW => id_object == OBJID_WINDOW.0 && id_child == CHILDID_SELF as i32,
+        EVENT_OBJECT_REORDER => true,
+        _ => false,
+    };
+    if relevant {
+        // `try_borrow` skips an event raised from inside the callback.
+        Z_ORDER_CALLBACK.with(|slot| {
+            let Ok(callback) = slot.try_borrow() else {
+                return;
+            };
+            if let Some(callback) = callback.as_ref() {
+                callback();
+            }
+        });
+    }
+}
+
 /// Hide a bubble (see [`show_no_activate`]).
 pub fn hide(window: &tauri::WebviewWindow) -> io::Result<()> {
     let hwnd = hwnd_of(window)?;