feat: Windows taskbar overlay badge via ITaskbarList3

Adds set_badge_count Tauri command (Windows only). Uses CoCreateInstance
to get ITaskbarList3 and SetOverlayIcon to display a dynamically drawn
badge on the taskbar button. The badge is a 16x16 GDI bitmap: red circle,
white bold Segoe UI text, capped at "99+". Passing count=0 clears the
overlay. Uses windows = 0.61 (already a transitive dep via webview2-com).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/Cargo.toml

@@ -44,6 +44,12 @@ tauri-plugin-updater = "2"
 
 [target.'cfg(target_os = "windows")'.dependencies]
 webview2-com = "0.38"
+windows = { version = "0.61", features = [
+    "Win32_Graphics_Gdi",
+    "Win32_System_Com",
+    "Win32_UI_Shell",
+    "Win32_UI_WindowsAndMessaging",
+] }
 
 [lib]
 name = "app_lib"

src-tauri/src/lib.rs

@@ -6,12 +6,146 @@
 use tauri::{webview::{NewWindowResponse, WebviewWindowBuilder}, WebviewUrl};
 use tauri_plugin_opener::OpenerExt;
 
+#[tauri::command]
+fn set_badge_count(count: u32, window: tauri::Window) -> Result<(), String> {
+    #[cfg(target_os = "windows")]
+    {
+        use windows::{
+            core::PCWSTR,
+            Win32::{
+                Foundation::{BOOL, COLORREF, HWND, RECT},
+                Graphics::Gdi::{
+                    CreateCompatibleDC, CreateDIBSection, CreatePen, CreateSolidBrush,
+                    DeleteDC, DeleteObject, DIB_RGB_COLORS, DrawTextW, Ellipse,
+                    ReleaseDC, SelectObject, SetBkMode, SetTextColor, BITMAPINFO,
+                    BITMAPINFOHEADER, BI_RGB, DT_CENTER, DT_SINGLELINE, DT_VCENTER,
+                    PS_NULL, TRANSPARENT, CreateFontW, DEFAULT_CHARSET, DEFAULT_PITCH,
+                    DEFAULT_QUALITY, CLIP_DEFAULT_PRECIS, FW_BOLD, FF_DONTCARE,
+                    OUT_DEFAULT_PRECIS,
+                },
+                UI::{
+                    Shell::{ITaskbarList3, TaskbarList},
+                    WindowsAndMessaging::{CreateBitmap, CreateIconIndirect, DestroyIcon, ICONINFO},
+                },
+                System::Com::{CoCreateInstance, CLSCTX_INPROC_SERVER},
+            },
+        };
+
+        let hwnd = HWND(window.hwnd().map_err(|e| e.to_string())?.0 as _);
+
+        let hicon = if count > 0 {
+            let label = if count > 99 {
+                "99+".to_string()
+            } else {
+                count.to_string()
+            };
+            let label_wide: Vec<u16> = label.encode_utf16().chain(std::iter::once(0)).collect();
+
+            unsafe {
+                let size = 16i32;
+                let hdc_screen = windows::Win32::Graphics::Gdi::GetDC(HWND(std::ptr::null_mut()));
+                let hdc = CreateCompatibleDC(hdc_screen);
+
+                let bmi = BITMAPINFO {
+                    bmiHeader: BITMAPINFOHEADER {
+                        biSize: std::mem::size_of::<BITMAPINFOHEADER>() as u32,
+                        biWidth: size,
+                        biHeight: -size,
+                        biPlanes: 1,
+                        biBitCount: 32,
+                        biCompression: BI_RGB.0,
+                        ..Default::default()
+                    },
+                    ..Default::default()
+                };
+                let mut bits: *mut std::ffi::c_void = std::ptr::null_mut();
+                let hbm_color = CreateDIBSection(hdc, &bmi, DIB_RGB_COLORS, &mut bits, None, 0)
+                    .map_err(|e| e.to_string())?;
+                let old_bm = SelectObject(hdc, hbm_color);
+
+                // Red circle (COLORREF is 0x00BBGGRR)
+                let hbrush = CreateSolidBrush(COLORREF(0x003030DD));
+                let old_brush = SelectObject(hdc, hbrush);
+                let hpen = CreatePen(PS_NULL, 0, COLORREF(0));
+                let old_pen = SelectObject(hdc, hpen);
+                let _ = Ellipse(hdc, 0, 0, size, size);
+
+                // White bold text centered
+                let hfont = CreateFontW(
+                    11, 0, 0, 0, FW_BOLD.0 as i32, 0, 0, 0,
+                    DEFAULT_CHARSET.0 as u32,
+                    OUT_DEFAULT_PRECIS.0 as u32,
+                    CLIP_DEFAULT_PRECIS.0 as u32,
+                    DEFAULT_QUALITY.0 as u32,
+                    (DEFAULT_PITCH.0 | FF_DONTCARE.0) as u32,
+                    windows::core::w!("Segoe UI"),
+                );
+                let old_font = SelectObject(hdc, hfont);
+                SetTextColor(hdc, COLORREF(0x00FFFFFF));
+                let _ = SetBkMode(hdc, TRANSPARENT);
+                let mut rect = RECT { left: 0, top: 0, right: size, bottom: size };
+                let _ = DrawTextW(
+                    hdc,
+                    &label_wide[..label_wide.len() - 1],
+                    &mut rect,
+                    DT_CENTER | DT_VCENTER | DT_SINGLELINE,
+                );
+
+                SelectObject(hdc, old_brush);
+                SelectObject(hdc, old_pen);
+                SelectObject(hdc, old_font);
+                SelectObject(hdc, old_bm);
+                let _ = DeleteObject(hbrush);
+                let _ = DeleteObject(hpen);
+                let _ = DeleteObject(hfont);
+
+                let hbm_mask = CreateBitmap(size, size, 1, 1, None)
+                    .map_err(|e| { let _ = DeleteObject(hbm_color); e.to_string() })?;
+
+                let icon_info = ICONINFO {
+                    fIcon: BOOL(1),
+                    xHotspot: 0,
+                    yHotspot: 0,
+                    hbmMask: hbm_mask,
+                    hbmColor: hbm_color,
+                };
+                let hicon = CreateIconIndirect(&icon_info)
+                    .map_err(|e| { let _ = DeleteObject(hbm_color); let _ = DeleteObject(hbm_mask); e.to_string() })?;
+
+                let _ = DeleteObject(hbm_color);
+                let _ = DeleteObject(hbm_mask);
+                let _ = DeleteDC(hdc);
+                let _ = ReleaseDC(HWND(std::ptr::null_mut()), hdc_screen);
+
+                Some(hicon)
+            }
+        } else {
+            None
+        };
+
+        unsafe {
+            let taskbar: ITaskbarList3 =
+                CoCreateInstance(&TaskbarList, None, CLSCTX_INPROC_SERVER)
+                    .map_err(|e| e.to_string())?;
+            taskbar.HrInit().map_err(|e| e.to_string())?;
+            taskbar
+                .SetOverlayIcon(hwnd, hicon, PCWSTR::null())
+                .map_err(|e| e.to_string())?;
+            if let Some(icon) = hicon {
+                let _ = DestroyIcon(icon);
+            }
+        }
+    }
+    Ok(())
+}
+
 pub fn run() {
     let port: u16 = 44548;
     let context = tauri::generate_context!();
     let builder = tauri::Builder::default();
 
     builder
+        .invoke_handler(tauri::generate_handler![set_badge_count])
         .plugin(tauri_plugin_localhost::Builder::new(port).build())
         .plugin(tauri_plugin_window_state::Builder::default().build())
         .plugin(tauri_plugin_opener::init())