[fix] improve Windows taskbar icon clarity at small sizes

apps/desktop/src/gui/application/gui_application.cpp

@@ -40,6 +40,7 @@
 
 #include "platform/windows/gui/slint_backend.h"
 #include "platform/windows/gui/tray/win_tray.h"
+#include "platform/windows/gui/window_icons.h"
 #elif defined(__APPLE__)
 #include "platform/macos/gui/tray/mac_tray.h"
 #include "platform/window_drag.h"
@@ -1013,6 +1014,9 @@ int GuiApplication::Run() {
   // when deciding whether the last hidden window should stop the event loop.
   // Keep the loop alive until the tray's explicit Exit action requests quit.
   ui_->main->show();
+#if _WIN32
+  ConfigureWindowsWindowIcons(ui_->main->window().win32_hwnd());
+#endif
   slint::run_event_loop(slint::EventLoopMode::RunUntilQuit);
   Cleanup();
   return 0;
@@ -1155,6 +1159,9 @@ void GuiApplication::InitializeSystemTray() {
       return;
     }
     ui_->main->show();
+#if _WIN32
+    ConfigureWindowsWindowIcons(ui_->main->window().win32_hwnd());
+#endif
     ui_->main->window().set_minimized(false);
 #if defined(__APPLE__)
     MacActivateWindow(ui_->main->window().appkit_view());
@@ -2162,6 +2169,9 @@ void GuiApplication::SyncMainWindow() {
   if (!ui_) {
     return;
   }
+#if _WIN32
+  ConfigureWindowsWindowIcons(ui_->main->window().win32_hwnd());
+#endif
 #if defined(__APPLE__)
   if (ui_->main_native_titlebar_attempts > 0) {
     if (HideDisabledMainWindowZoomButton()) {
@@ -2451,6 +2461,9 @@ void GuiApplication::SyncStreamWindow() {
   if (!ui_->stream) {
     return;
   }
+#if _WIN32
+  ConfigureWindowsWindowIcons((*ui_->stream)->window().win32_hwnd());
+#endif
   (*ui_->stream)->set_window_maximized((*ui_->stream)->window().is_maximized());
 #if defined(__APPLE__)
   if (ui_->stream_live_resize_configuration_attempts > 0) {
@@ -2895,6 +2908,10 @@ void GuiApplication::SyncServerWindow() {
     return;
   }
 
+#if _WIN32
+  ConfigureWindowsWindowIcons((*ui_->server)->window().win32_hwnd());
+#endif
+
   std::vector<ui::ControllerEntry> controllers;
   std::vector<slint::SharedString> names;
   ui_->controller_ids.clear();

apps/desktop/src/platform/windows/gui/window_icons.cpp

@@ -0,0 +1,100 @@
+#include "window_icons.h"
+
+#include <commctrl.h>
+
+#include <memory>
+#include <utility>
+
+namespace crossdesk {
+namespace {
+
+constexpr UINT_PTR kIconSubclassId = 1;
+
+struct WindowIcons {
+  HICON small_icon = nullptr;
+  HICON large_icon = nullptr;
+
+  ~WindowIcons() {
+    if (small_icon) DestroyIcon(small_icon);
+    if (large_icon) DestroyIcon(large_icon);
+  }
+
+  bool Load(UINT dpi) {
+    const HMODULE module = GetModuleHandleW(nullptr);
+    small_icon = static_cast<HICON>(LoadImageW(
+        module, L"IDI_ICON1", IMAGE_ICON,
+        GetSystemMetricsForDpi(SM_CXSMICON, dpi),
+        GetSystemMetricsForDpi(SM_CYSMICON, dpi), 0));
+    large_icon = static_cast<HICON>(LoadImageW(
+        module, L"IDI_ICON1", IMAGE_ICON,
+        GetSystemMetricsForDpi(SM_CXICON, dpi),
+        GetSystemMetricsForDpi(SM_CYICON, dpi), 0));
+    return small_icon && large_icon;
+  }
+
+  void Apply(HWND window) const {
+    // Do not destroy the replaced handles: Slint/winit owns those images.
+    SendMessageW(window, WM_SETICON, ICON_SMALL,
+                 reinterpret_cast<LPARAM>(small_icon));
+    SendMessageW(window, WM_SETICON, ICON_BIG,
+                 reinterpret_cast<LPARAM>(large_icon));
+  }
+};
+
+LRESULT CALLBACK IconSubclassProc(HWND window, UINT message, WPARAM wparam,
+                                  LPARAM lparam, UINT_PTR subclass_id,
+                                  DWORD_PTR reference_data) {
+  auto* icons = reinterpret_cast<WindowIcons*>(reference_data);
+  if (message == WM_SETICON) {
+    // Slint supplies the same PNG for both sizes, including during deferred
+    // window initialization. Keep using the matching frames from our ICO.
+    if (wparam == ICON_SMALL) {
+      lparam = reinterpret_cast<LPARAM>(icons->small_icon);
+    } else if (wparam == ICON_BIG) {
+      lparam = reinterpret_cast<LPARAM>(icons->large_icon);
+    }
+    return DefSubclassProc(window, message, wparam, lparam);
+  }
+  if (message == WM_DPICHANGED) {
+    WindowIcons next;
+    if (next.Load(HIWORD(wparam))) {
+      std::swap(icons->small_icon, next.small_icon);
+      std::swap(icons->large_icon, next.large_icon);
+      icons->Apply(window);
+      // 'next' releases the previous pair only after both were replaced.
+    }
+  }
+  if (message == WM_NCDESTROY) {
+    const LRESULT result = DefSubclassProc(window, message, wparam, lparam);
+    RemoveWindowSubclass(window, IconSubclassProc, subclass_id);
+    delete icons;
+    return result;
+  }
+  return DefSubclassProc(window, message, wparam, lparam);
+}
+
+}  // namespace
+
+bool ConfigureWindowsWindowIcons(HWND window) {
+  if (!window) {
+    return false;
+  }
+  DWORD_PTR existing = 0;
+  if (GetWindowSubclass(window, IconSubclassProc, kIconSubclassId, &existing)) {
+    return true;
+  }
+  auto icons = std::make_unique<WindowIcons>();
+  const UINT dpi = GetDpiForWindow(window);
+  if (!dpi || !icons->Load(dpi) ||
+      !SetWindowSubclass(window, IconSubclassProc, kIconSubclassId,
+                         reinterpret_cast<DWORD_PTR>(icons.get()))) {
+    return false;
+  }
+  // The subclass owns the pair until WM_NCDESTROY, even if Slint hides and
+  // recreates its native window while the component itself stays alive.
+  auto* installed = icons.release();
+  installed->Apply(window);
+  return true;
+}
+
+}  // namespace crossdesk

apps/desktop/src/platform/windows/gui/window_icons.h

@@ -0,0 +1,14 @@
+#ifndef CROSSDESK_WINDOWS_WINDOW_ICONS_H_
+#define CROSSDESK_WINDOWS_WINDOW_ICONS_H_
+
+#include <windows.h>
+
+namespace crossdesk {
+
+// Call on the window's UI thread once its native handle exists. Repeated calls
+// are harmless. Icons follow DPI changes and are released with the window.
+bool ConfigureWindowsWindowIcons(HWND window);
+
+}  // namespace crossdesk
+
+#endif  // CROSSDESK_WINDOWS_WINDOW_ICONS_H_

apps/desktop/xmake/targets.lua

@@ -399,8 +399,10 @@ function setup_targets()
         if is_os("windows") then
             add_cxxflags("/bigobj")
             add_links("opengl32")
+            add_syslinks("Comctl32", {public = true})
             add_files("apps/desktop/src/platform/windows/gui/tray/win_tray.cpp",
                 "apps/desktop/src/platform/windows/gui/slint_backend.cpp",
+                "apps/desktop/src/platform/windows/gui/window_icons.cpp",
                 "apps/desktop/src/platform/windows/gui/slint_renderer_probe.cpp",
                 "apps/desktop/src/platform/common/gui/opengl_video_renderer.cpp",
                 "apps/desktop/src/platform/common/gui/cuda_driver.cpp",