diff --git a/shiny/driver/internal/win32/syscall_windows.go b/shiny/driver/internal/win32/syscall_windows.go
index 6c55058..53857c4 100644
--- a/shiny/driver/internal/win32/syscall_windows.go
+++ b/shiny/driver/internal/win32/syscall_windows.go
@@ -178,5 +178,6 @@
 //sys   _PostQuitMessage(exitCode int32) = user32.PostQuitMessage
 //sys	_RegisterClass(wc *_WNDCLASS) (atom uint16, err error) = user32.RegisterClassW
 //sys	_ShowWindow(hwnd syscall.Handle, cmdshow int32) (wasvisible bool) = user32.ShowWindow
+//sys	_ScreenToClient(hwnd syscall.Handle, lpPoint *_POINT) (ok bool) = user32.ScreenToClient
 //sys   _ToUnicodeEx(wVirtKey uint32, wScanCode uint32, lpKeyState *byte, pwszBuff *uint16, cchBuff int32, wFlags uint32, dwhkl syscall.Handle) (ret int32) = user32.ToUnicodeEx
 //sys	_TranslateMessage(msg *_MSG) (done bool) = user32.TranslateMessage
diff --git a/shiny/driver/internal/win32/win32.go b/shiny/driver/internal/win32/win32.go
index 6b70162..78895bd 100644
--- a/shiny/driver/internal/win32/win32.go
+++ b/shiny/driver/internal/win32/win32.go
@@ -183,6 +183,15 @@
 		// distinct beginning and end. Should the intermediate events be
 		// DirNone?
 		e.Direction = mouse.DirStep
+
+		// Convert from screen to window coordinates.
+		p := _POINT{
+			int32(e.X),
+			int32(e.Y),
+		}
+		_ScreenToClient(hwnd, &p)
+		e.X = float32(p.X)
+		e.Y = float32(p.Y)
 	default:
 		panic("sendMouseEvent() called on non-mouse message")
 	}
diff --git a/shiny/driver/internal/win32/zsyscall_windows.go b/shiny/driver/internal/win32/zsyscall_windows.go
index 3ff0582..07430ac 100644
--- a/shiny/driver/internal/win32/zsyscall_windows.go
+++ b/shiny/driver/internal/win32/zsyscall_windows.go
@@ -11,6 +11,31 @@
 
 var _ unsafe.Pointer
 
+// Do the interface allocations only once for common
+// Errno values.
+const (
+	errnoERROR_IO_PENDING = 997
+)
+
+var (
+	errERROR_IO_PENDING error = syscall.Errno(errnoERROR_IO_PENDING)
+)
+
+// errnoErr returns common boxed Errno values, to prevent
+// allocations at runtime.
+func errnoErr(e syscall.Errno) error {
+	switch e {
+	case 0:
+		return nil
+	case errnoERROR_IO_PENDING:
+		return errERROR_IO_PENDING
+	}
+	// TODO: add more here, after collecting data on the common
+	// error values see on Windows. (perhaps when running
+	// all.bat?)
+	return e
+}
+
 var (
 	moduser32 = windows.NewLazySystemDLL("user32.dll")
 
@@ -32,6 +57,7 @@
 	procPostQuitMessage   = moduser32.NewProc("PostQuitMessage")
 	procRegisterClassW    = moduser32.NewProc("RegisterClassW")
 	procShowWindow        = moduser32.NewProc("ShowWindow")
+	procScreenToClient    = moduser32.NewProc("ScreenToClient")
 	procToUnicodeEx       = moduser32.NewProc("ToUnicodeEx")
 	procTranslateMessage  = moduser32.NewProc("TranslateMessage")
 )
@@ -41,7 +67,7 @@
 	dc = syscall.Handle(r0)
 	if dc == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -53,7 +79,7 @@
 	r1, _, e1 := syscall.Syscall(procReleaseDC.Addr(), 2, uintptr(hwnd), uintptr(dc), 0)
 	if r1 == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -72,7 +98,7 @@
 	hwnd = syscall.Handle(r0)
 	if hwnd == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -90,7 +116,7 @@
 	r1, _, e1 := syscall.Syscall(procDestroyWindow.Addr(), 1, uintptr(hwnd), 0, 0)
 	if r1 == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -108,7 +134,7 @@
 	r1, _, e1 := syscall.Syscall(procGetClientRect.Addr(), 2, uintptr(hwnd), uintptr(unsafe.Pointer(rect)), 0)
 	if r1 == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -126,7 +152,7 @@
 	r1, _, e1 := syscall.Syscall(procGetKeyboardState.Addr(), 1, uintptr(unsafe.Pointer(lpKeyState)), 0, 0)
 	if r1 == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -145,7 +171,7 @@
 	ret = int32(r0)
 	if ret == -1 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -158,7 +184,7 @@
 	cursor = syscall.Handle(r0)
 	if cursor == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -171,7 +197,7 @@
 	icon = syscall.Handle(r0)
 	if icon == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -195,7 +221,7 @@
 	atom = uint16(r0)
 	if atom == 0 {
 		if e1 != 0 {
-			err = error(e1)
+			err = errnoErr(e1)
 		} else {
 			err = syscall.EINVAL
 		}
@@ -209,6 +235,12 @@
 	return
 }
 
+func _ScreenToClient(hwnd syscall.Handle, lpPoint *_POINT) (ok bool) {
+	r0, _, _ := syscall.Syscall(procScreenToClient.Addr(), 2, uintptr(hwnd), uintptr(unsafe.Pointer(lpPoint)), 0)
+	ok = r0 != 0
+	return
+}
+
 func _ToUnicodeEx(wVirtKey uint32, wScanCode uint32, lpKeyState *byte, pwszBuff *uint16, cchBuff int32, wFlags uint32, dwhkl syscall.Handle) (ret int32) {
 	r0, _, _ := syscall.Syscall9(procToUnicodeEx.Addr(), 7, uintptr(wVirtKey), uintptr(wScanCode), uintptr(unsafe.Pointer(lpKeyState)), uintptr(unsafe.Pointer(pwszBuff)), uintptr(cchBuff), uintptr(wFlags), uintptr(dwhkl), 0, 0)
 	ret = int32(r0)