win32 — pure-Go bindings to the Windows API

github.com/go-mswin/win32 is the owned, pure-Go CGO=0 foundation for calling the Windows API from Go — the peer of go-macos/objc. It targets 64-bit Windows (amd64 + arm64); "win32" is the Windows API name, not a bitness (there is no separate "Win64 API").

Built on golang.org/x/sys/windows (no cgo). Provides the shared surface the fleet currently hand-rolls in more than one place:

• lazy DLL + proc binding for user32 / gdi32 / kernel32 / advapi32 / shell32 / combase / shcore
• a hidden message window + GetMessage/TranslateMessage/DispatchMessage pump
• WNDCLASSEXW / MSG / rect / point types and the common CreateWindowExW, RegisterClassExW, DefWindowProcW, LoadCursorW, PostQuitMessage, …
• top-down 32-bpp BGRA StretchDIBits blit helper
• device contexts and GDI objects: GetDC / GetWindowDC / ReleaseDC, CreateCompatibleDC / DeleteDC, SelectObject / DeleteObject, BitBlt / StretchBlt / PatBlt, SetStretchBltMode, GetDeviceCaps
• callback-driven enumeration: EnumDisplayMonitors and EnumWindows, each with ONE process-wide trampoline. windows.NewCallback allocates out of a pool the runtime caps at 2000 and going past it is runtime.throw, not a recoverable panic — so a wrapper that builds its callback per call puts a hard ceiling on how many times an application may ask, and then kills it
• display enumeration: GetMonitorInfo (MONITORINFOEXW, cbSize set for you — a wrong one is silently rejected), EnumDisplayDevices / DisplayAdapters / DisplayMonitors for the adapter and panel names, GetDpiForMonitor off shcore, MonitorFromWindow, and SetProcessDPIAwarenessContext with the Per-Monitor-V2 context — without which every rectangle the process reads is virtualised and plausible
• window state and geometry: ShowWindow, UpdateWindow, InvalidateRect, SetWindowPos, SetForegroundWindow, GetForegroundWindow, GetWindowRect, GetClientRect, IsWindow, IsWindowVisible, IsIconic, GetWindowText, GetClassName, GetWindowLongPtr / SetWindowLongPtr (which x/sys/windows does not wrap), GetWindowThreadProcessID

Consumers

go-widgets/tray, go-widgets/window (win32 backend), go-mswin/screencapture, and the WinRT plumbing in the weft Windows apps — one owned binding instead of duplicated hand-rolls.

Verification

The live glue is proven on a real Windows machine and the record is committed: win32-vm-proof-2026-08-13.txt (message pump, BGRA blit) and win32-display-vm-proof-2026-08-25.txt (display enumeration, compared field by field against System.Windows.Forms.Screen and dxdiag).

The callback ceiling is guarded by an ungated test — TestEnumerationsDoNotConsumeACallbackPerCall runs 3000 walks of each enumeration on every Windows CI lane. It needs no desktop. With a trampoline per call it does not fail; the test binary dies partway through, which is the shape of the defect.

License

BSD-3-Clause — copyright the go-mswin authors.