Register a system-wide hotkey (Win+Alt+C) on the hidden owner window that calls the same ToggleCalculator() path as a tray-icon click, so the window can be shown/hidden from any app. MOD_NOREPEAT prevents repeated toggles while the keys are held; if the combo is already taken the registration fails silently and the tray click keeps working. Bump the SDK target from Vista (0x0600) to Windows 10/11 (0x0A00) so MOD_NOREPEAT and other modern APIs are available, and drop the obsolete _WIN32_WINDOWS (Win9x) define. Also bump VER_BUILD to 2. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>