ipn/desktop: fix panics on Windows 10, x86

[G,S]etWindowLongPtrW are not available on 32-bit Windows, where [G,S]etWindowLongW should be used instead.
The initial revision of tailscale#14945 imported the win package for calling and other Win32 API functions, which exported
the correct API depending on the platform. However, the same logic wasn't implemented when we removed
the win package dependency in a later revision, resulting in panics on Windows 10 x86 (there's no 32-bit Windows 11).

In this PR, we update the ipn/desktop package to use either [G,S]etWindowLongPtrW or [G,S]etWindowLongW
depending on the platform.

Fixes tailscale#15684

Signed-off-by: Nick Khyl <nickk@tailscale.com>

(cherry picked from commit 60614fa)

Author: nickkhyl

ipn/desktop/mksyscall.go

@@ -11,8 +11,6 @@ package desktop
 //sys registerClassEx(windowClass *_WNDCLASSEX) (atom uint16, err error) [atom==0] = user32.RegisterClassExW
 //sys createWindowEx(dwExStyle uint32, lpClassName *uint16, lpWindowName *uint16, dwStyle uint32, x int32, y int32, nWidth int32, nHeight int32, hWndParent windows.HWND, hMenu windows.Handle, hInstance windows.Handle, lpParam unsafe.Pointer) (hWnd windows.HWND, err error) [hWnd==0] = user32.CreateWindowExW
 //sys defWindowProc(hwnd windows.HWND, msg uint32, wparam uintptr, lparam uintptr) (res uintptr) = user32.DefWindowProcW
-//sys setWindowLongPtr(hwnd windows.HWND, index int32, newLong uintptr) (res uintptr, err error) [res==0 && e1!=0] = user32.SetWindowLongPtrW
-//sys getWindowLongPtr(hwnd windows.HWND, index int32) (res uintptr, err error) [res==0 && e1!=0] = user32.GetWindowLongPtrW
 //sys sendMessage(hwnd windows.HWND, msg uint32, wparam uintptr, lparam uintptr) (res uintptr) = user32.SendMessageW
 //sys getMessage(lpMsg *_MSG, hwnd windows.HWND, msgMin uint32, msgMax uint32) (ret int32) = user32.GetMessageW
 //sys translateMessage(lpMsg *_MSG) (res bool) = user32.TranslateMessage

ipn/desktop/sessions_windows.go

@@ -670,3 +670,38 @@ func (cs _WTS_CONNECTSTATE_CLASS) ToSessionStatus() SessionStatus {
 		return ClosedSession
 	}
 }
+
+var (
+	procGetWindowLongPtrW *windows.LazyProc
+	procSetWindowLongPtrW *windows.LazyProc
+)
+
+func init() {
+	// GetWindowLongPtrW and SetWindowLongPtrW are only available on 64-bit platforms.
+	// https://web.archive.org/web/20250414195520/https://learn.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-getwindowlongptrw
+	if runtime.GOARCH == "386" || runtime.GOARCH == "arm" {
+		procGetWindowLongPtrW = moduser32.NewProc("GetWindowLongW")
+		procSetWindowLongPtrW = moduser32.NewProc("SetWindowLongW")
+	} else {
+		procGetWindowLongPtrW = moduser32.NewProc("GetWindowLongPtrW")
+		procSetWindowLongPtrW = moduser32.NewProc("SetWindowLongPtrW")
+	}
+}
+
+func getWindowLongPtr(hwnd windows.HWND, index int32) (res uintptr, err error) {
+	r0, _, e1 := syscall.Syscall(procGetWindowLongPtrW.Addr(), 2, uintptr(hwnd), uintptr(index), 0)
+	res = uintptr(r0)
+	if res == 0 && e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+func setWindowLongPtr(hwnd windows.HWND, index int32, newLong uintptr) (res uintptr, err error) {
+	r0, _, e1 := syscall.Syscall(procSetWindowLongPtrW.Addr(), 3, uintptr(hwnd), uintptr(index), uintptr(newLong))
+	res = uintptr(r0)
+	if res == 0 && e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}