fix(app): set Windows taskbar and Alt+Tab big icon (ICON_BIG) (#3818) (#4681)

apps/pdfcraft/Cargo.toml

@@ -64,6 +64,10 @@ windows = { version = "0.62", default-features = false, features = ["std", "UI_V
 # socket. `std` has no Unix sockets on Windows, and calling Winsock directly needs unsafe, which
 # our workspace forbids. Already in the lockfile through zbus; pinned exactly, review before bumping.
 uds_windows = "=1.2.1"
+# Full-resolution taskbar and Alt+Tab icon (`ICON_BIG`, issue #3818). Set both window icons
+# directly so eframe cannot replace ICON_BIG with a system-sized copy. Already in the tree
+# through eframe and covered by the workspace's winit patch.
+winit = { version = "0.30", default-features = false }
 
 [build-dependencies]
 # Icon + VERSIONINFO for pdfcraft.exe; build.rs only uses it when targeting Windows.

apps/pdfcraft/src/main.rs

@@ -340,7 +340,15 @@ fn native_options(integrated: bool, app_frame: bool, renderer: eframe::Renderer)
         .with_drag_and_drop(true)
         // Wayland app id: matches packaging/linux/ai.storyteller.pdfcraft.desktop.
         .with_app_id(APP_ID);
+    // eframe replaces ICON_BIG with a system-sized icon on its first active frame.
+    // An explicitly empty icon disables that setter (None selects eframe's default).
+    // Windows installs both sizes directly through winit in app_creator instead.
+    #[cfg(target_os = "windows")]
+    {
+        viewport = viewport.with_icon(egui::IconData::default());
+    }
     // Dock, taskbar, Alt-Tab and launcher icon when running unbundled.
+    #[cfg(not(target_os = "windows"))]
     match eframe::icon_data::from_png_bytes(APP_ICON_PNG) {
         Ok(icon) => viewport = viewport.with_icon(icon),
         Err(e) => log::warn!("app icon: {e}"),
@@ -391,6 +399,48 @@ fn native_wayland(_: &eframe::CreationContext<'_>) -> bool {
     false
 }
 
+/// Decode `png` as a [`winit::window::Icon`] for the Windows taskbar and Alt+Tab (`ICON_BIG`).
+#[cfg(target_os = "windows")]
+fn taskbar_icon(png: &[u8]) -> Option<winit::window::Icon> {
+    let icon = match eframe::icon_data::from_png_bytes(png) {
+        Ok(icon) => icon,
+        Err(e) => {
+            log::warn!("taskbar icon: {e}");
+            return None;
+        }
+    };
+    match winit::window::Icon::from_rgba(icon.rgba, icon.width, icon.height) {
+        Ok(icon) => Some(icon),
+        Err(e) => {
+            log::warn!("taskbar icon: {e}");
+            None
+        }
+    }
+}
+
+/// Install the decoded [`winit::window::Icon`] from `png` via `set` when decoding succeeds.
+#[cfg(target_os = "windows")]
+fn install_taskbar_icon(png: &[u8], set: impl FnOnce(winit::window::Icon)) {
+    if let Some(icon) = taskbar_icon(png) {
+        set(icon);
+    }
+}
+
+/// Install both window icons before the first frame. eframe's icon setter is disabled in
+/// `native_options`, so it cannot replace the full-resolution taskbar/Alt+Tab icon (#3818).
+#[cfg(target_os = "windows")]
+fn set_taskbar_icon(window: &winit::window::Window) {
+    use winit::platform::windows::{IconExtWindows as _, WindowExtWindows as _};
+    install_taskbar_icon(APP_ICON_PNG, |icon| window.set_taskbar_icon(Some(icon)));
+    // The title bar's icon: the small image drawn for this scale in the exe's own icon (build.rs).
+    let side = (16.0 * window.scale_factor()).round().clamp(16.0, 256.0) as u32;
+    match winit::window::Icon::from_resource(1, Some(winit::dpi::PhysicalSize::new(side, side))) {
+        Ok(icon) => window.set_window_icon(Some(icon)),
+        Err(e) => log::warn!("window icon: {e}"),
+    }
+}
+
+/// Build the [`eframe::AppCreator`] closure that initializes [`PdfCraftApp`] once the window and renderer are ready.
 fn app_creator<'a>(
     launch: Launch,
     started: Rc<Cell<bool>>,
@@ -418,6 +468,9 @@ fn app_creator<'a>(
         // it is set to On or Off.
         #[cfg(target_os = "windows")]
         {
+            if let Some(window) = cc.winit_window() {
+                set_taskbar_icon(window);
+            }
             app.motion.source = Some(motion::source());
         }
         app.set_startup_update_check(true);

apps/pdfcraft/src/tests.rs

@@ -137,6 +137,31 @@ fn the_desktop_frame_is_turned_off_only_when_the_app_draws_one() {
     assert_eq!(native_options(false, true, eframe::Renderer::Glow).viewport.decorations, Some(false));
 }
 
+#[test]
+fn the_window_and_windows_taskbar_icons_load_from_the_bundled_png() {
+    let icon = eframe::icon_data::from_png_bytes(super::APP_ICON_PNG).expect("the bundled PNG decodes as the window icon");
+    assert!(icon.width > 0 && icon.height > 0);
+    assert_eq!(icon.rgba.len(), (icon.width as usize) * (icon.height as usize) * 4);
+    #[cfg(not(target_os = "windows"))]
+    assert_eq!(super::native_options(false, false, eframe::Renderer::Glow).viewport.icon.as_deref(), Some(&icon));
+    #[cfg(target_os = "windows")]
+    {
+        // None would load eframe's default icon; only the explicit empty sentinel prevents
+        // its first-frame setter from overwriting our full-resolution ICON_BIG.
+        for renderer in [eframe::Renderer::Glow, eframe::Renderer::Wgpu] {
+            let viewport = super::native_options(false, false, renderer).viewport;
+            assert_eq!(viewport.icon.as_deref(), Some(&egui::IconData::default()));
+        }
+        assert_eq!((icon.width, icon.height), (256, 256));
+        let mut installed = None;
+        super::install_taskbar_icon(super::APP_ICON_PNG, |icon| installed = Some(icon));
+        assert!(installed.is_some(), "issue #3818: ICON_BIG loads and installs from APP_ICON_PNG");
+        let mut bad = None;
+        super::install_taskbar_icon(b"not a png", |icon| bad = Some(icon));
+        assert!(bad.is_none());
+    }
+}
+
 #[test]
 fn a_software_renderer_is_shown_to_the_user() {
     use eframe::wgpu::DeviceType;