app: [Windows] make custom decorated windows behave like regular windows

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

app/internal/windows/windows.go

@@ -245,6 +245,7 @@ const (
 	WM_MOUSEHWHEEL          = 0x020E
 	WM_NCACTIVATE           = 0x0086
 	WM_NCHITTEST            = 0x0084
+	WM_NCCALCSIZE           = 0x0083
 	WM_PAINT                = 0x000F
 	WM_QUIT                 = 0x0012
 	WM_SETCURSOR            = 0x0020

app/os_windows.go

@@ -325,6 +325,11 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 		// The system destroys the HWND for us.
 		w.hwnd = 0
 		windows.PostQuitMessage(0)
+	case windows.WM_NCCALCSIZE:
+		if !w.config.Decorated {
+			// No client areas; we draw decorations ourselves.
+			return 0
+		}
 	case windows.WM_PAINT:
 		w.draw(true)
 	case windows.WM_SIZE:
@@ -669,9 +674,6 @@ func (w *window) Configure(options []Option) {
 	swpStyle := uintptr(windows.SWP_NOZORDER | windows.SWP_FRAMECHANGED)
 	winStyle := uintptr(windows.WS_OVERLAPPEDWINDOW)
 	style &^= winStyle
-	if !w.config.Decorated {
-		winStyle = 0
-	}
 	switch w.config.Mode {
 	case Minimized:
 		style |= winStyle
@@ -695,9 +697,12 @@ func (w *window) Configure(options []Option) {
 		// Set desired window size.
 		wr.Right = wr.Left + width
 		wr.Bottom = wr.Top + height
-		// Convert from client size to window size.
+		// Compute client size and position. Note that the client size is
+		// equal to the window size when we are in control of decorations.
 		r := wr
-		windows.AdjustWindowRectEx(&r, uint32(style), 0, dwExStyle)
+		if w.config.Decorated {
+			windows.AdjustWindowRectEx(&r, uint32(style), 0, dwExStyle)
+		}
 		// Calculate difference between client and full window sizes.
 		w.deltas.width = r.Right - wr.Right + wr.Left - r.Left
 		w.deltas.height = r.Bottom - wr.Bottom + wr.Top - r.Top