app: [Windows] suppress double-click behaviour for custom decorations

Fixes: https://todo.sr.ht/~eliasnaur/gio/600
Signed-off-by: Elias Naur <mail@eliasnaur.com>

app/internal/windows/windows.go

@@ -266,6 +266,7 @@ const (
 	WM_MOUSEWHEEL           = 0x020A
 	WM_MOUSEHWHEEL          = 0x020E
 	WM_NCACTIVATE           = 0x0086
+	WM_NCLBUTTONDBLCLK      = 0x00A3
 	WM_NCHITTEST            = 0x0084
 	WM_NCCALCSIZE           = 0x0083
 	WM_PAINT                = 0x000F

app/os_windows.go

@@ -326,6 +326,12 @@ func windowProc(hwnd syscall.Handle, msg uint32, wParam, lParam uintptr) uintptr
 		mi := windows.GetMonitorInfo(w.hwnd)
 		szp.Rgrc[0] = mi.WorkArea
 		return 0
+	case windows.WM_NCLBUTTONDBLCLK:
+		if !w.config.Decorated {
+			// Override Windows behaviour when we
+			// draw decorations.
+			return 0
+		}
 	case windows.WM_PAINT:
 		w.draw(true)
 	case windows.WM_SIZE: