app: [Windows] support resize gestures for undecorated windows

We're about to remove the client-controlled system.Action machinery
for resize gestures initiated by the client. This is the replacement
implementation for Windows, where the behaviour is hard-coded to
mimic the decorated automatic handling.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/windows/windows.go

@@ -113,7 +113,15 @@ const (
 
 	HWND_TOPMOST = ^(uint32(1) - 1) // -1
 
-	HTCLIENT = 1
+	HTCLIENT      = 1
+	HTLEFT        = 10
+	HTRIGHT       = 11
+	HTTOP         = 12
+	HTTOPLEFT     = 13
+	HTTOPRIGHT    = 14
+	HTBOTTOM      = 15
+	HTBOTTOMLEFT  = 16
+	HTBOTTOMRIGHT = 17
 
 	IDC_APPSTARTING = 32650 // Standard arrow and small hourglass
 	IDC_ARROW       = 32512 // Standard arrow
@@ -146,13 +154,16 @@ const (
 
 	SCS_SETSTR = GCS_COMPREADSTR | GCS_COMPSTR
 
+	SM_CXSIZEFRAME = 32
+	SM_CYSIZEFRAME = 33
+
 	SW_SHOWDEFAULT   = 10
 	SW_SHOWMINIMIZED = 2
 	SW_SHOWMAXIMIZED = 3
 	SW_SHOWNORMAL    = 1
 	SW_SHOW          = 5
-	SWP_FRAMECHANGED = 0x0020
 
+	SWP_FRAMECHANGED  = 0x0020
 	SWP_NOMOVE        = 0x0002
 	SWP_NOOWNERZORDER = 0x0200
 	SWP_NOSIZE        = 0x0001
@@ -231,6 +242,7 @@ const (
 	WM_MOUSEMOVE            = 0x0200
 	WM_MOUSEWHEEL           = 0x020A
 	WM_MOUSEHWHEEL          = 0x020E
+	WM_NCHITTEST            = 0x0084
 	WM_PAINT                = 0x000F
 	WM_QUIT                 = 0x0012
 	WM_SETCURSOR            = 0x0020
@@ -316,6 +328,7 @@ var (
 	_GetMessage                  = user32.NewProc("GetMessageW")
 	_GetMessageTime              = user32.NewProc("GetMessageTime")
 	_GetMonitorInfo              = user32.NewProc("GetMonitorInfoW")
+	_GetSystemMetrics            = user32.NewProc("GetSystemMetrics")
 	_GetWindowLong               = user32.NewProc("GetWindowLongPtrW")
 	_GetWindowLong32             = user32.NewProc("GetWindowLongW")
 	_GetWindowPlacement          = user32.NewProc("GetWindowPlacement")
@@ -499,6 +512,11 @@ func GetMessageTime() time.Duration {
 	return time.Duration(r) * time.Millisecond
 }
 
+func GetSystemMetrics(nIndex int) int {
+	r, _, _ := _GetSystemMetrics.Call(uintptr(nIndex))
+	return int(r)
+}
+
 // GetWindowDPI returns the effective DPI of the window.
 func GetWindowDPI(hwnd syscall.Handle) int {
 	// Check for GetDpiForWindow, introduced in Windows 10.

app/os_windows.go

@@ -54,8 +54,9 @@ type window struct {
 
 	animating bool
 
-	deltas winDeltas
-	config Config
+	deltas     winDeltas
+	borderSize image.Point
+	config     Config
 }
 
 const _WM_WAKEUP = windows.WM_USER + iota
@@ -210,6 +211,11 @@ func (w *window) update() {
 		}
 	}
 	w.config.Size = size
+
+	w.borderSize = image.Pt(
+		windows.GetSystemMetrics(windows.SM_CXSIZEFRAME),
+		windows.GetSystemMetrics(windows.SM_CYSIZEFRAME),
+	)
 	w.w.Event(ConfigEvent{Config: w.config})
 }
 
@@ -279,6 +285,15 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 		w.w.Event(key.FocusEvent{Focus: true})
 	case windows.WM_KILLFOCUS:
 		w.w.Event(key.FocusEvent{Focus: false})
+	case windows.WM_NCHITTEST:
+		if w.config.Decorated || w.config.Mode != Windowed {
+			// Let the system handle it.
+			break
+		}
+		x, y := coordsFromlParam(lParam)
+		np := windows.Point{X: int32(x), Y: int32(y)}
+		windows.ScreenToClient(w.hwnd, &np)
+		return w.hitTest(int(np.X), int(np.Y))
 	case windows.WM_MOUSEMOVE:
 		x, y := coordsFromlParam(lParam)
 		p := f32.Point{X: float32(x), Y: float32(y)}
@@ -411,6 +426,37 @@ func getModifiers() key.Modifiers {
 	return kmods
 }
 
+// hitTest returns the non-client area hit by the point, needed to
+// process WM_NCHITTEST.
+func (w *window) hitTest(x, y int) uintptr {
+	top := y <= w.borderSize.Y
+	bottom := y >= w.config.Size.Y-w.borderSize.Y
+	left := x <= w.borderSize.X
+	right := x >= w.config.Size.X-w.borderSize.X
+	switch {
+	default:
+		fallthrough
+	case !top && !bottom && !left && !right:
+		return windows.HTCLIENT
+	case top && left:
+		return windows.HTTOPLEFT
+	case top && right:
+		return windows.HTTOPRIGHT
+	case bottom && left:
+		return windows.HTBOTTOMLEFT
+	case bottom && right:
+		return windows.HTBOTTOMRIGHT
+	case top:
+		return windows.HTTOP
+	case bottom:
+		return windows.HTBOTTOM
+	case left:
+		return windows.HTLEFT
+	case right:
+		return windows.HTRIGHT
+	}
+}
+
 func (w *window) pointerButton(btn pointer.Buttons, press bool, lParam uintptr, kmods key.Modifiers) {
 	var typ pointer.Type
 	if press {