diff --git a/date_windows.go b/date_windows.go
index 92a886b..898b9f3 100644
--- a/date_windows.go
+++ b/date_windows.go
@@ -146,25 +146,25 @@ func (dlg *calendarDialog) layout(dpi dpi) {
 	}
 }
 
-func calendarProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
+func calendarProc(wnd win.HWND, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
 	var dlg *calendarDialog
 	switch msg {
 	case win.WM_NCCREATE:
-		saveBackRef(wnd, *lparam)
+		saveBackRef(uintptr(wnd), *lparam)
 		dlg = (*calendarDialog)(*lparam)
 	case win.WM_NCDESTROY:
-		deleteBackRef(wnd)
+		deleteBackRef(uintptr(wnd))
 	default:
-		dlg = (*calendarDialog)(loadBackRef(wnd))
+		dlg = (*calendarDialog)(loadBackRef(uintptr(wnd)))
 	}
 
 	switch msg {
 	case win.WM_DESTROY:
-		postQuitMessage.Call(0)
+		win.PostQuitMessage(0)
 
 	case win.WM_CLOSE:
 		dlg.err = ErrCanceled
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_COMMAND:
 		switch wparam {
@@ -179,14 +179,13 @@ func calendarProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointe
 		case win.IDNO:
 			dlg.err = ErrExtraButton
 		}
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_DPICHANGED:
 		dlg.layout(dpi(uint32(wparam) >> 16))
 
 	default:
-		res, _, _ := defWindowProc.Call(wnd, uintptr(msg), wparam, uintptr(unsafe.Pointer(lparam)))
-		return res
+		return win.DefWindowProc(wnd, msg, wparam, unsafe.Pointer(lparam))
 	}
 
 	return 0
diff --git a/entry_windows.go b/entry_windows.go
index 64b491d..aad8f79 100644
--- a/entry_windows.go
+++ b/entry_windows.go
@@ -140,25 +140,25 @@ func (dlg *entryDialog) layout(dpi dpi) {
 	}
 }
 
-func entryProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
+func entryProc(wnd win.HWND, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
 	var dlg *entryDialog
 	switch msg {
 	case win.WM_NCCREATE:
-		saveBackRef(wnd, *lparam)
+		saveBackRef(uintptr(wnd), *lparam)
 		dlg = (*entryDialog)(*lparam)
 	case win.WM_NCDESTROY:
-		deleteBackRef(wnd)
+		deleteBackRef(uintptr(wnd))
 	default:
-		dlg = (*entryDialog)(loadBackRef(wnd))
+		dlg = (*entryDialog)(loadBackRef(uintptr(wnd)))
 	}
 
 	switch msg {
 	case win.WM_DESTROY:
-		postQuitMessage.Call(0)
+		win.PostQuitMessage(0)
 
 	case win.WM_CLOSE:
 		dlg.err = ErrCanceled
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_COMMAND:
 		switch wparam {
@@ -171,14 +171,13 @@ func entryProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointer)
 		case win.IDNO:
 			dlg.err = ErrExtraButton
 		}
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_DPICHANGED:
 		dlg.layout(dpi(uint32(wparam) >> 16))
 
 	default:
-		res, _, _ := defWindowProc.Call(wnd, uintptr(msg), wparam, uintptr(unsafe.Pointer(lparam)))
-		return res
+		return win.DefWindowProc(wnd, msg, wparam, unsafe.Pointer(lparam))
 	}
 
 	return 0
diff --git a/internal/win/user32.go b/internal/win/user32.go
index 337b7c7..b2aec2f 100644
--- a/internal/win/user32.go
+++ b/internal/win/user32.go
@@ -174,13 +174,16 @@ type WNDCLASSEX struct {
 //sys CreateIconFromResource(resBits []byte, icon bool, ver uint32) (ret Handle, err error) = user32.CreateIconFromResource
 //sys CreateWindowEx(exStyle uint32, className *uint16, windowName *uint16, style uint32, x int, y int, width int, height int, parent HWND, menu Handle, instance Handle, param unsafe.Pointer) (ret HWND, err error) = user32.CreateWindowExW
 //sys DestroyIcon(icon Handle) (err error) = user32.DestroyIcon
+//sys DestroyWindow(wnd HWND) (err error) = user32.DestroyWindow
 //sys DispatchMessage(msg *MSG) (ret uintptr) = user32.DispatchMessageW
+//sys DefWindowProc(wnd HWND, msg uint32, wparam uintptr, lparam unsafe.Pointer) (ret uintptr) = user32.DefWindowProcW
 //sys EnableWindow(wnd HWND, enable bool) (ok bool) = user32.EnableWindow
 //sys EnumChildWindows(parent HWND, enumFunc uintptr, lparam unsafe.Pointer) = user32.EnumChildWindows
 //sys EnumWindows(enumFunc uintptr, lparam unsafe.Pointer) (err error) = user32.EnumChildWindows
 //sys GetDlgCtrlID(wnd HWND) (ret int) = user32.GetDlgCtrlID
 //sys getDpiForWindow(wnd HWND) (ret int) = user32.GetDpiForWindow
 //sys GetMessage(msg *MSG, wnd HWND, msgFilterMin uint32, msgFilterMax uint32) (ret uintptr) = user32.GetMessageW
+//sys GetSystemMetrics(index int) (ret int) = user32.GetSystemMetrics
 //sys GetWindowDC(wnd HWND) (ret Handle) = user32.GetWindowDC
 //sys GetWindowRect(wnd HWND, cmdShow *RECT) (err error) = user32.GetWindowRect
 //sys GetWindowText(wnd HWND, str *uint16, maxCount int) (ret int, err error) = user32.GetWindowTextW
@@ -188,6 +191,7 @@ type WNDCLASSEX struct {
 //sys IsDialogMessage(wnd HWND, msg *MSG) (ok bool) = user32.IsDialogMessageW
 //sys LoadIcon(instance Handle, resource uintptr) (ret Handle, err error) = user32.LoadIconW
 //sys LoadImage(instance Handle, name *uint16, typ int, cx int, cy int, load int) (ret Handle, err error) = user32.LoadImageW
+//sys PostQuitMessage(exitCode int) = user32.PostQuitMessage
 //sys RegisterClassEx(cls *WNDCLASSEX) (err error) = user32.RegisterClassExW
 //sys ReleaseDC(wnd HWND, dc Handle) (ok bool) = user32.ReleaseDC
 //sys SendMessage(wnd HWND, msg uint32, wparam uintptr, lparam uintptr) (ret uintptr) = user32.SendMessageW
diff --git a/internal/win/zsyscall_windows.go b/internal/win/zsyscall_windows.go
index 3d55843..107e986 100644
--- a/internal/win/zsyscall_windows.go
+++ b/internal/win/zsyscall_windows.go
@@ -68,13 +68,16 @@ var (
 	procShell_NotifyIconW            = modshell32.NewProc("Shell_NotifyIconW")
 	procCreateIconFromResource       = moduser32.NewProc("CreateIconFromResource")
 	procCreateWindowExW              = moduser32.NewProc("CreateWindowExW")
+	procDefWindowProcW               = moduser32.NewProc("DefWindowProcW")
 	procDestroyIcon                  = moduser32.NewProc("DestroyIcon")
+	procDestroyWindow                = moduser32.NewProc("DestroyWindow")
 	procDispatchMessageW             = moduser32.NewProc("DispatchMessageW")
 	procEnableWindow                 = moduser32.NewProc("EnableWindow")
 	procEnumChildWindows             = moduser32.NewProc("EnumChildWindows")
 	procGetDlgCtrlID                 = moduser32.NewProc("GetDlgCtrlID")
 	procGetDpiForWindow              = moduser32.NewProc("GetDpiForWindow")
 	procGetMessageW                  = moduser32.NewProc("GetMessageW")
+	procGetSystemMetrics             = moduser32.NewProc("GetSystemMetrics")
 	procGetWindowDC                  = moduser32.NewProc("GetWindowDC")
 	procGetWindowRect                = moduser32.NewProc("GetWindowRect")
 	procGetWindowTextLengthW         = moduser32.NewProc("GetWindowTextLengthW")
@@ -82,6 +85,7 @@ var (
 	procIsDialogMessageW             = moduser32.NewProc("IsDialogMessageW")
 	procLoadIconW                    = moduser32.NewProc("LoadIconW")
 	procLoadImageW                   = moduser32.NewProc("LoadImageW")
+	procPostQuitMessage              = moduser32.NewProc("PostQuitMessage")
 	procRegisterClassExW             = moduser32.NewProc("RegisterClassExW")
 	procReleaseDC                    = moduser32.NewProc("ReleaseDC")
 	procSendMessageW                 = moduser32.NewProc("SendMessageW")
@@ -253,6 +257,12 @@ func CreateWindowEx(exStyle uint32, className *uint16, windowName *uint16, style
 	return
 }
 
+func DefWindowProc(wnd HWND, msg uint32, wparam uintptr, lparam unsafe.Pointer) (ret uintptr) {
+	r0, _, _ := syscall.Syscall6(procDefWindowProcW.Addr(), 4, uintptr(wnd), uintptr(msg), uintptr(wparam), uintptr(lparam), 0, 0)
+	ret = uintptr(r0)
+	return
+}
+
 func DestroyIcon(icon Handle) (err error) {
 	r1, _, e1 := syscall.Syscall(procDestroyIcon.Addr(), 1, uintptr(icon), 0, 0)
 	if r1 == 0 {
@@ -261,6 +271,14 @@ func DestroyIcon(icon Handle) (err error) {
 	return
 }
 
+func DestroyWindow(wnd HWND) (err error) {
+	r1, _, e1 := syscall.Syscall(procDestroyWindow.Addr(), 1, uintptr(wnd), 0, 0)
+	if r1 == 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
 func DispatchMessage(msg *MSG) (ret uintptr) {
 	r0, _, _ := syscall.Syscall(procDispatchMessageW.Addr(), 1, uintptr(unsafe.Pointer(msg)), 0, 0)
 	ret = uintptr(r0)
@@ -308,6 +326,12 @@ func GetMessage(msg *MSG, wnd HWND, msgFilterMin uint32, msgFilterMax uint32) (r
 	return
 }
 
+func GetSystemMetrics(index int) (ret int) {
+	r0, _, _ := syscall.Syscall(procGetSystemMetrics.Addr(), 1, uintptr(index), 0, 0)
+	ret = int(r0)
+	return
+}
+
 func GetWindowDC(wnd HWND) (ret Handle) {
 	r0, _, _ := syscall.Syscall(procGetWindowDC.Addr(), 1, uintptr(wnd), 0, 0)
 	ret = Handle(r0)
@@ -364,6 +388,11 @@ func LoadImage(instance Handle, name *uint16, typ int, cx int, cy int, load int)
 	return
 }
 
+func PostQuitMessage(exitCode int) {
+	syscall.Syscall(procPostQuitMessage.Addr(), 1, uintptr(exitCode), 0, 0)
+	return
+}
+
 func RegisterClassEx(cls *WNDCLASSEX) (err error) {
 	r1, _, e1 := syscall.Syscall(procRegisterClassExW.Addr(), 1, uintptr(unsafe.Pointer(cls)), 0, 0)
 	if r1 == 0 {
diff --git a/list_windows.go b/list_windows.go
index e5f95aa..cb8d54c 100644
--- a/list_windows.go
+++ b/list_windows.go
@@ -160,25 +160,25 @@ func (dlg *listDialog) layout(dpi dpi) {
 	}
 }
 
-func listProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
+func listProc(wnd win.HWND, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
 	var dlg *listDialog
 	switch msg {
 	case win.WM_NCCREATE:
-		saveBackRef(wnd, *lparam)
+		saveBackRef(uintptr(wnd), *lparam)
 		dlg = (*listDialog)(*lparam)
 	case win.WM_NCDESTROY:
-		deleteBackRef(wnd)
+		deleteBackRef(uintptr(wnd))
 	default:
-		dlg = (*listDialog)(loadBackRef(wnd))
+		dlg = (*listDialog)(loadBackRef(uintptr(wnd)))
 	}
 
 	switch msg {
 	case win.WM_DESTROY:
-		postQuitMessage.Call(0)
+		win.PostQuitMessage(0)
 
 	case win.WM_CLOSE:
 		dlg.err = ErrCanceled
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_COMMAND:
 		switch wparam {
@@ -208,14 +208,13 @@ func listProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointer) u
 		case win.IDNO:
 			dlg.err = ErrExtraButton
 		}
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_DPICHANGED:
 		dlg.layout(dpi(uint32(wparam) >> 16))
 
 	default:
-		res, _, _ := defWindowProc.Call(wnd, uintptr(msg), wparam, uintptr(unsafe.Pointer(lparam)))
-		return res
+		return win.DefWindowProc(wnd, msg, wparam, unsafe.Pointer(lparam))
 	}
 
 	return 0
diff --git a/progress_windows.go b/progress_windows.go
index a58f62f..b4b9efe 100644
--- a/progress_windows.go
+++ b/progress_windows.go
@@ -234,25 +234,25 @@ func (d *progressDialog) layout(dpi dpi) {
 	}
 }
 
-func progressProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
+func progressProc(wnd win.HWND, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
 	var dlg *progressDialog
 	switch msg {
 	case win.WM_NCCREATE:
-		saveBackRef(wnd, *lparam)
+		saveBackRef(uintptr(wnd), *lparam)
 		dlg = (*progressDialog)(*lparam)
 	case win.WM_NCDESTROY:
-		deleteBackRef(wnd)
+		deleteBackRef(uintptr(wnd))
 	default:
-		dlg = (*progressDialog)(loadBackRef(wnd))
+		dlg = (*progressDialog)(loadBackRef(uintptr(wnd)))
 	}
 
 	switch msg {
 	case win.WM_DESTROY:
-		postQuitMessage.Call(0)
+		win.PostQuitMessage(0)
 
 	case win.WM_CLOSE:
 		dlg.err = ErrCanceled
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_COMMAND:
 		switch wparam {
@@ -265,14 +265,13 @@ func progressProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointe
 		case win.IDNO:
 			dlg.err = ErrExtraButton
 		}
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_DPICHANGED:
 		dlg.layout(dpi(uint32(wparam) >> 16))
 
 	default:
-		res, _, _ := defWindowProc.Call(wnd, uintptr(msg), wparam, uintptr(unsafe.Pointer(lparam)))
-		return res
+		return win.DefWindowProc(wnd, msg, wparam, unsafe.Pointer(lparam))
 	}
 
 	return 0
diff --git a/pwd_windows.go b/pwd_windows.go
index 1da90be..84d71f4 100644
--- a/pwd_windows.go
+++ b/pwd_windows.go
@@ -161,25 +161,25 @@ func (dlg *passwordDialog) layout(dpi dpi) {
 	}
 }
 
-func passwordProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
+func passwordProc(wnd win.HWND, msg uint32, wparam uintptr, lparam *unsafe.Pointer) uintptr {
 	var dlg *passwordDialog
 	switch msg {
 	case win.WM_NCCREATE:
-		saveBackRef(wnd, *lparam)
+		saveBackRef(uintptr(wnd), *lparam)
 		dlg = (*passwordDialog)(*lparam)
 	case win.WM_NCDESTROY:
-		deleteBackRef(wnd)
+		deleteBackRef(uintptr(wnd))
 	default:
-		dlg = (*passwordDialog)(loadBackRef(wnd))
+		dlg = (*passwordDialog)(loadBackRef(uintptr(wnd)))
 	}
 
 	switch msg {
 	case win.WM_DESTROY:
-		postQuitMessage.Call(0)
+		win.PostQuitMessage(0)
 
 	case win.WM_CLOSE:
 		dlg.err = ErrCanceled
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_COMMAND:
 		switch wparam {
@@ -193,14 +193,13 @@ func passwordProc(wnd uintptr, msg uint32, wparam uintptr, lparam *unsafe.Pointe
 		case win.IDNO:
 			dlg.err = ErrExtraButton
 		}
-		destroyWindow.Call(wnd)
+		win.DestroyWindow(wnd)
 
 	case win.WM_DPICHANGED:
 		dlg.layout(dpi(uint32(wparam) >> 16))
 
 	default:
-		res, _, _ := defWindowProc.Call(wnd, uintptr(msg), wparam, uintptr(unsafe.Pointer(lparam)))
-		return res
+		return win.DefWindowProc(wnd, msg, wparam, unsafe.Pointer(lparam))
 	}
 
 	return 0
diff --git a/util_windows.go b/util_windows.go
index 28cbe0f..a7be33a 100644
--- a/util_windows.go
+++ b/util_windows.go
@@ -20,10 +20,6 @@ var (
 	user32 = windows.NewLazySystemDLL("user32.dll")
 
 	callNextHookEx       = user32.NewProc("CallNextHookEx")
-	defWindowProc        = user32.NewProc("DefWindowProcW")
-	destroyWindow        = user32.NewProc("DestroyWindow")
-	getSystemMetrics     = user32.NewProc("GetSystemMetrics")
-	postQuitMessage      = user32.NewProc("PostQuitMessage")
 	setWindowsHookEx     = user32.NewProc("SetWindowsHookExW")
 	systemParametersInfo = user32.NewProc("SystemParametersInfoW")
 	unhookWindowsHookEx  = user32.NewProc("UnhookWindowsHookEx")
@@ -312,16 +308,11 @@ func (i *icon) delete() {
 }
 
 func centerWindow(wnd win.HWND) {
-	getMetric := func(i uintptr) int32 {
-		n, _, _ := getSystemMetrics.Call(i)
-		return int32(n)
-	}
-
 	var rect win.RECT
 	win.GetWindowRect(wnd, &rect)
-	x := (getMetric(0 /* SM_CXSCREEN */) - (rect.Right - rect.Left)) / 2
-	y := (getMetric(1 /* SM_CYSCREEN */) - (rect.Bottom - rect.Top)) / 2
-	win.SetWindowPos(wnd, 0, int(x), int(y), 0, 0, 0x5) // SWP_NOZORDER|SWP_NOSIZE
+	x := (win.GetSystemMetrics(0 /* SM_CXSCREEN */) - int(rect.Right-rect.Left)) / 2
+	y := (win.GetSystemMetrics(1 /* SM_CYSCREEN */) - int(rect.Bottom-rect.Top)) / 2
+	win.SetWindowPos(wnd, 0, x, y, 0, 0, 0x5) // SWP_NOZORDER|SWP_NOSIZE
 }
 
 func getWindowString(wnd win.HWND) string {